MIPS: ath79: add common WMAC device for AR913X based boards

Add common platform_device and helper code to make the registration
of the built-in wireless MAC easier on the Atheros AR9130/AR9132
based boards. Also register the WMAC device on the AR81 board.

Signed-off-by: Gabor Juhos <juhosg@openwrt.org>
Cc: linux-mips@linux-mips.org
Cc: Imre Kaloz <kaloz@openwrt.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/1962/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>