MIPS: ath79: Add OF support to the GPIO driver

Replace the simple GPIO chip registration by a platform driver
and make ath79_gpio_init() just register the device.

Signed-off-by: Alban Bedel <albeu@free.fr>
Cc: linux-mips@linux-mips.org
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>