imx: mxs: reimplement get_cpu_rev

Rewrite get_cpu_rev, from "static const char *get_cpu_rev(void)" to
"u32 get_cpu_rev(void)". To align with get_cpu_rev of other i.MXes.

Also write get_imx_type to replace get_cpu_type, since we have
macro named get_cpu_type.

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