omap: Beagle: only Cx boards use pin 23 for write protect

system_rev comes from u-boot and is a constant 0x20, so
Bx boards also fall in this 'if' and will get setup with the
wrong gpio_wp pin. Switch to using the Beagle revision routine
to correcly set pin 23 only for C1/2/3 and C4 Boards. Bx boards
will then use the correct default pin setting.

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