imx: vf610 add get_cpu_rev

Since we need to support runtime check for different drivers, we need
to add get_cpu_rev for vf610, otherwise there will be build errors.

This patch introduces a dummy CPU id which is not read from chip
silicon. Later when we can get the real id from chip, can fix the
value of MXC_CPU_VF610 then.

Signed-off-by: Peng Fan <Peng.Fan@freescale.com>
Suggested-by: Stefano Babic <sbabic@denx.de>
Cc: Stefano Babic <sbabic@denx.de>
Acked-by: Stefano Babic <sbabic@denx.de>