soc: rp2350: support PWM channels >8

RP2350 adds four more PWM slices from the eight available on RP2040,
which are only broken out to package pins on RP2350B. This change fixes
the driver to support the correct number of slices on RP2350.

Tested by confirming that PWM can correctly be configured on GPIO 44 of
RP2350B.

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