imx: mx31 use new formula for get_cpu_rev
Use new formula for get_cpu_rev, since we need to use this formula
to do runtime check for all i.MXes.
Signed-off-by: Peng Fan <Peng.Fan@freescale.com>
Cc: Stefano Babic <sbabic@denx.de>
Reviewed-by: Stefano Babic <sbabic@denx.de>