MIPS: ath79: add GPIOLIB support

This patch implements generic GPIO routines for the built-in
GPIO controllers of the Atheros AR71XX/AR724X/AR913X SoCs.

Signed-off-by: Gabor Juhos <juhosg@openwrt.org>
Signed-off-by: Imre Kaloz <kaloz@openwrt.org>
Cc: David Brownell <dbrownell@users.sourceforge.net>
Cc: linux-mips@linux-mips.org
Cc: Luis R. Rodriguez <lrodriguez@atheros.com>
Cc: Cliff Holden <Cliff.Holden@Atheros.com>
Cc: Kathy Giori <Kathy.Giori@Atheros.com>
Patchwork: https://patchwork.linux-mips.org/patch/1948/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>