rp2: Add HSTX ALT function.

[sfe-SparkFro]

Summary

Follow up from #17692. Adds the HSTX alternate pin function for GPIO12-19 on the RP2350.

Testing

Tested with an RP2350B board, and seems to work fine!

>>> machine.Pin(12, mode=machine.Pin.ALT, alt=machine.Pin.ALT_HSTX)
Pin(GPIO12, mode=ALT, alt=HSTX)

[github-actions]

Code size report:

Reference:  shared/runtime/pyexec: Set PYEXEC_FORCED_EXIT flag for SystemExit. [a686410]
Comparison: rp2: Add HSTX ALT function. [merge of 7a554db]
  mpy-cross:    +0 +0.000% 
   bare-arm:    +0 +0.000% 
minimal x86:    +0 +0.000% 
   unix x64:    +0 +0.000% standard
      stm32:    +0 +0.000% PYBV10
     mimxrt:    +0 +0.000% TEENSY40
        rp2:    +0 +0.000% RPI_PICO_W
       samd:    +0 +0.000% ADAFRUIT_ITSYBITSY_M4_EXPRESS
  qemu rv32:    +0 +0.000% VIRT_RV32

[sfe-SparkFro]

Forgot that af_idx is now increased by one, affecting this spot too, fixed with 7a554db!

[dpgeorge]

Thanks, this looks good.

I tested on a RPI_PICO2_W, and the HSTX function could be selected correctly. And other existing alt functions seemed to be correct.

I also tested building RPI_PICO and found that the firmware is unchanged with this PR, which is as expected (binary equivalent except for the changing git hash that's embedded in the firmware).

[dpgeorge]

Rebased and merged in 1ef76c7