soc: rp2350: correctly handle GPIOs >31

RP2350 has 48 GPIOs, where only the first 30 are broken out to pins on
RP2350A (same as RP2040) and the remaining 18 are only usable on RP2350B.
This change makes the soc pinctrl driver support GPIOs above 31, where
previously it was impossible to configure GPIOs 32 through 47.

Tested on RP2350B, confirming that GPIO44 can be correctly configured for
PWM.

Signed-off-by: Peter Marheine <peter@taricorp.net>