MIPS: ath79: add common GPIO LEDs device

MIPS: ath79: add common GPIO LEDs device

Almost all boards have one or more LEDs connected to GPIO lines. This
patch adds common code to register a platform_device for them.

The patch also adds support for the LEDs on the PB44 board.

Signed-off-by: Gabor Juhos <juhosg@openwrt.org>
Signed-off-by: Imre Kaloz <kaloz@openwrt.org>
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/1953/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>