gpio: ep93xx: Switch A and B to use GPIOLIB_IRQCHIP

We can quite easily switch banks/ports A and B to use
GPIOLIB_IRQCHIP which is code that will be more careful
about handling interrupt descriptors and use a proper
irqdomain for translating the IRQs. This cuts down some
code in favor of using the implementation inside
gpiolib.

Acked-by: Alexander Sverdlin <alexander.sverdlin@gmail.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>