gpio: pcal64xxa: switch to 8 bit internal data automatically

Convert the internal uint16_t data of the driver to an internal type
that automatically switches from uint8_t to uint16_t depending on
whether any 16 bit device is present in the system or not. This shrinks
the internal structures by few bytes when the extra data is not needed.

Signed-off-by: Fabio Baltieri <fabiobaltieri@google.com>