[ARM] 5262/2: [AT91] Support for GPIO-connected buttons on SAM9260-EK board

Add support or the GPIO-connected buttons on the Atmel AT91SAM9260-EK board.

Signed-off-by: Sedji Gaouaou <sedji.gaouaou@atmel.com>
Signed-off-by: Andrew Victor <linux@maxim.org.za>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>