gpio: ath79: Convert to the state container design pattern

Turn the ath79 driver into a true driver supporting multiple
instances. While at it also removed unneed includes and make use of
the BIT() macro.

Signed-off-by: Alban Bedel <albeu@free.fr>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>