ARM OMAP3: RX-51 board - add initialization of gpio keys

Initialize some of the RX-51 input GPIO lines as gpio keys. Enable gpio
keys as a module in rx51_defconfig.

Signed-off-by: Jani Nikula <ext-jani.1.nikula@nokia.com>
Acked-by: Tony Lindgren <tony@atomide.com>
Signed-off-by: Dmitry Torokhov <dtor@mail.ru>