Prepare LPSPI driver for embedded-hal 1.0 rework

[Finomnis]

Some fixes/modifications to the existing LPSPI driver.

This is in preparation for the embedded-hal 1.0 rework, which will introduce a new high level driver that wraps this low level one.

Changes:

• Remove CS_PIN from Spi device
  • Rationale: eh1 primarily uses software CS pins. There is an extension for hardware CS pin support, but I think we should add it later as a special case instead of by default. This will simplify API design a lot.
• Split Spi into Spi + SpiCsPin in the board subcrate
• Add initial state to gpio::Output constructor. This is important for cases where the state needs to be set before enabling the pin.
• Fix/rework check for valid frame sizes (existing implementation did not cover all corner cases)
• Rework SPI clock initialization
  • Make sure we never get a baud rate faster than specified (slower is pretty much always ok for peripherals)
  • Configure PCSSCK and SCKPCS properly for contiguous transfers
• Some improvements in reset logic
• Add soft_reset that cancels the current transfer without requiring a full re-initialization
• Move more values into enqueue_transaction so that multiple transactions with different settings can be enqueued in series
• Add flush function that waits for the current transaction to be finished whilst repeatedly checking for errors
• Fix disabled() not waiting for the current transfer to be finished
• Take set_watermark out of disabled, because it doesn't require it.

[Finomnis]

@mciantyre Sorry that I was gone for a while. Don't know how much time I will be able to spend on the SPI device, but I'll try to do a little :)

I finally finished this PR. I didn't update examples yet, but I think there's no point really in updating them until we have a properly reworked driver.

Any feedback?

[Finomnis]

@mciantyre Ping :)

[teburd]

I’d say squash this and I’ll merge, changes look good. Ian is presumably busy with other endeavors at the moment.

[Finomnis]

@teburd I don't think I agree time to continue this any time soon, so let's keep that one open because it's a breaking change.

[mciantyre]

There's useful non-breaking changes in this branch, changes that could help today's users. I extracted them into #156.

[Finomnis]

@mciantyre Welcome back! Glad to see your life has cooled down enough to dabble in some Rust again :)