gpio: realtek-otto: Add RTL931x support

The RTL931x SoC series has support for 32 GPIOs, although not all lines
may be broken out to a physical pad.

The GPIO bank's parent interrupt can be routed to either or both of the
SoC's CPU cores by the GIC. Line-by-line IRQ balancing is not possible
on these SoCs.

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