boards: thingy91x: Fix external flash issue with mcuboot

[maxd-nordic]

This patch fixes the issue that prevented MCUboot
to work at all on the Thingy:91 X.

[NordicBuilder]

Test specification

CI/Jenkins/NRF

• Integration Platforms

CI/Jenkins/integration

Test Module	File based changes	Manually selected	West overwrite

Detailed information of selected test modules

Note: This message is automatically posted and updated by the CI

[NordicBuilder]

Memory footprint analysis revealed the following potential issues

applications.nrf_desktop.zdebug_mcuboot_smp.nrf52840dk[nrf52840dk/nrf52840]: ROM size increased by 1216[B] in comparison to the main[f0ca215] branch. - link (cc: @MarekPieta)
applications.nrf_desktop.zrelease_fast_pair.keyboard[nrf52kbd/nrf52832]: ROM size increased by 1216[B] in comparison to the main[f0ca215] branch. - link (cc: @MarekPieta)
applications.nrf_desktop.zdebug[nrf52840dk/nrf52840]: ROM size increased by 1216[B] in comparison to the main[f0ca215] branch. - link (cc: @MarekPieta)
applications.nrf_desktop.zdebug[nrf52840gmouse/nrf52840]: ROM size increased by 1216[B] in comparison to the main[f0ca215] branch. - link (cc: @MarekPieta)
applications.nrf_desktop.zdebug_nrf21540ek[nrf52840dk/nrf52840]: ROM size increased by 1256[B] in comparison to the main[f0ca215] branch. - link (cc: @MarekPieta)
applications.nrf_desktop.zdebug_wwcb[nrf52840dk/nrf52840]: ROM size increased by 1216[B] in comparison to the main[f0ca215] branch. - link (cc: @MarekPieta)
applications.nrf_desktop.zdebug_keyboard.usb_next[nrf52840dk/nrf52840]: ROM size increased by 1216[B] in comparison to the main[f0ca215] branch. - link (cc: @MarekPieta)
applications.nrf_desktop.zrelease.nrf52kbd[nrf52kbd/nrf52832]: ROM size increased by 1216[B] in comparison to the main[f0ca215] branch. - link (cc: @MarekPieta)
applications.nrf_desktop.zrelease[nrf52840gmouse/nrf52840]: ROM size increased by 1216[B] in comparison to the main[f0ca215] branch. - link (cc: @MarekPieta)
applications.nrf_desktop.zdebug.usb_next[nrf52840dk/nrf52840]: ROM size increased by 1216[B] in comparison to the main[f0ca215] branch. - link (cc: @MarekPieta)
applications.nrf_desktop.zdebug[nrf52kbd/nrf52832]: ROM size increased by 1216[B] in comparison to the main[f0ca215] branch. - link (cc: @MarekPieta)
applications.nrf_desktop.zdebug_keyboard[nrf52840dk/nrf52840]: ROM size increased by 1216[B] in comparison to the main[f0ca215] branch. - link (cc: @MarekPieta)
applications.nrf_desktop.zdebug_mcuboot_qspi[nrf52840dk/nrf52840]: ROM size increased by 1216[B] in comparison to the main[f0ca215] branch. - link (cc: @MarekPieta)
applications.nrf_desktop.zdebug.usb_next[nrf52840gmouse/nrf52840]: ROM size increased by 1216[B] in comparison to the main[f0ca215] branch. - link (cc: @MarekPieta)
applications.nrf_desktop.zdebug_mcuboot_smp.nrf52840gmouse[nrf52840gmouse/nrf52840]: ROM size increased by 1216[B] in comparison to the main[f0ca215] branch. - link (cc: @MarekPieta)
applications.nrf_desktop.zdebug_fast_pair.gmouse[nrf52840gmouse/nrf52840]: ROM size increased by 1216[B] in comparison to the main[f0ca215] branch. - link (cc: @MarekPieta)
applications.nrf_desktop.zdebugwithshell[nrf52840dk/nrf52840]: ROM size increased by 1216[B] in comparison to the main[f0ca215] branch. - link (cc: @MarekPieta)

Note: This message is automatically posted and updated by the CI (latest/sdk-nrf/PR-15733/1)