OMAP3: beagle: add support for beagleboard xM revision C

The USB enable GPIO has been in beagleboard xM revision C.
The USER button has been moved since beagleboard xM.
Also, board specific initialization has been moved to beagle_config struct
and initialized in omap3_beagle_init_rev. Default values in struct are for xMC.
Signed-off-by: NJoel A Fernandes <joelagnel@ti.com>
Signed-off-by: NKoen Kooi <koen@dominion.thruhere.net>
Acked-by: NJason Kridner <jkridner@beagleboard.org>
Signed-off-by: NTony Lindgren <tony@atomide.com>