gpio: ath79: Allow building in compile tests

To allow building the driver in compile tests we must drop the
dependency on asm/mach-ath79/ar71xx_regs.h. For this we replace the
include with local definition of the registers needed for this driver.

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