RP2040 SPI HAL Improvements

[haydenridd]

Relatively minor improvements, but the highlights:

• Minor renaming for clarity similar to my I2C PR
• Support for all the different frame formats the peripheral supports
• Support for all the different bit widths the peripheral supports
• TX FIFO preloading for performance improvements:

Example transaction before:

Example transaction with preloading (no gap!):

• Removed configuration of GPIO pins in peripheral driver
  • This actually had some immediate benefits, as the way the RP2040 peripheral controls the CSN pin automatically is a bit odd, it toggles CSN high after every byte when a GPIO is configured to .spi function:

• For the flash chip I tested this driver on, this would actually make it unusable was I forced to use this
• Instead, I manually drive CSN via a GPIO, which works perfectly well

• Not configuring the GPIOs within the peripheral driver gives users of the HAL this kind of flexibility

  The full test suite I wrote for this driver can be found here: https://github.com/haydenridd/microzig/tree/rp2040-spi

[haydenridd]

@ikskuh Requested changes applied!

[haydenridd]

For sure! I'll add an example then merge :)