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

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 <[email protected]>
Cc: [email protected]
Cc: Imre Kaloz <[email protected]>,
Cc: Luis R. Rodriguez <[email protected]>
Cc: Cliff Holden <[email protected]>
Cc: Kathy Giori <[email protected]>
Patchwork: https://patchwork.linux-mips.org/patch/1962/
Signed-off-by: Ralf Baechle <[email protected]>