drivers: gpio: fix pincm lut size

LUT sizes directly reflects the global data when enabled in dts
(even if no or few pins are really consumed). Also the PINCM
numbering across the series (g, l and c) is within 255, so fix
to use uint8_t to save the global space.

Signed-off-by: Parthiban Nallathambi <parthiban@linumiz.com>