Add 10xx FlexIO CCM configuration

[Florob]

This adds support for setting source clock and dividers for FlexIO clock roots in accordance with the RMs

This is WIP because I haven't gotten around to testing the current version on actual hardware yet, but I'd like feedback on the general approach.
The issue here is that FLEXIO1_CLK_ROOT on 1010/1020 is FLEXIO2_CLK_ROOT on 106x, so there is some magic involved in getting the module names and doc comments right. Due to this it might also make sense to move this into a separate module and re-export it.

[mciantyre]

I like it. The macros are convenient, and we could lower their invocation into the chip configuration modules, co-locating them with the other chip-specific CCM modules. I pushed a commit to show this, but we can throw it away.

[Florob]

I've finally gotten around to verifying this. After cursing at the PFD frequencies being wildly different from what is in the datasheet for a while I'm now reasonably confident that this is correct.

@mciantyre I agree with your changes and squashed them into the initial commit, as I don't think having them separate makes much sense.

[mciantyre]

Thanks for adding this. I rebased onto main, and I'll release in 0.5.4.