MIPS: ath79: add common GPIO buttons device

MIPS: ath79: add common GPIO buttons device

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

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

Signed-off-by: Gabor Juhos <[email protected]>
Signed-off-by: Imre Kaloz <[email protected]>
Cc: [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/1954/
Signed-off-by: Ralf Baechle <[email protected]>