Add support for new PIO v1 instruction features on RP2350:

• wait: add jmp_pin source (PINCTRL_JMP_PIN, src=3) with a
  2-bit offset argument.
• irq: add next_pio/prev_pio index modifiers for cross-PIO IRQ
  targeting.
• asm_pio: add in_count parameter encoding SHIFTCTRL.IN_COUNT
  (bits [4:0]) to mask unneeded IN-mapped pins to zero.
• asm_pio: fix fifo_join encoding to support RP2350 TXGET(4),
  TXPUT(8) and PUTGET(12) modes via SHIFTCTRL bits [15:14].

All changes are backward-compatible with RP2040 PIO v0.

Also update RP2 docs to cover new changes.

Signed-off-by: NED KONZ NED@METAMAGIX.TECH

Summary

The RP2350 added a number of improvements to the PIO hardware in the RP2040.

This PR is the first of three that will bring MicroPython's RP2 PIO support up to date.

Testing

Tested on a RPI_PICO2 board using #18974: all encoding tests passed,
covering new instructions, RP2350 FIFO join modes, in_count,
and regression checks for all existing instruction encodings.

I also tested on a RPI_PICO board and there were no regressions.

Generative AI

I used generative AI tools when creating this PR, but a human has checked the
code and is responsible for the code and the description above.