omap: Beagle: revision detection

Due to the omap3530 ES3.0 Silicon being used on both the
B5/B6 and C1/2/3 Beagle we can't use the cpu_is_omap34xx()
routines to differentiate the Beagle Boards.

However gpio pins 171,172,173 where setup for this prupose, so
lets use them.

Changes:
for older U-Boot's, use omap_mux_init_gpio()
keep Beagle Rev in board-omap3beagle.c
gpio_free on gpio request failure

Tested on Beagle Revisions: B5, C2, C4, and xMA

Signed-off-by: Robert Nelson <robertcnelson@gmail.com>
Acked-by: Jarkko Nikula <jhnikula@gmail.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>