OMAP3: Move get_cpu_rev() function to not duplicate code

OMAP3: Move get_cpu_rev() function to not duplicate code

The get_cpu_rev() function is implemented by various boards, this
patch moves this function to a common place to not duplicate code.

 board/omap3530beagle/omap3530beagle.c :159:u32 get_cpu_rev(void)
 board/igep00x0/igep00x0.c             :129:u32 get_cpu_rev(void)
 board/omap3430sdp/omap3430sdp.c       :82:u32 get_cpu_rev(void)
 board/omap3evm/omap3evm.c             :135:u32 get_cpu_rev(void)
 board/omap3430labrador/omap3430sdp.c  :79:u32 get_cpu_rev(void)
 board/overo/overo.c                   :164:u32 get_cpu_rev(void)

Signed-off-by: Enric Balletbo i Serra <eballetbo@gmail.com>
Signed-off-by: Anand Gadiyar <gadiyar@ti.com>