gpio: realtek-otto: Support reversed port layouts

The GPIO port layout on the RTL930x SoC series is reversed compared to
the RTL838x and RTL839x SoC series. Add new port offset calculator
functions to ensure the correct order is used when reading port IRQ
data, and ensure bgpio uses the right byte ordering.

Signed-off-by: Sander Vanheule <sander@svanheule.net>
Signed-off-by: Bartosz Golaszewski <brgl@bgdev.pl>