MX31: change return value of get_cpu_rev

Drop warnings in get_cpu_rev and changes the return value
(a u32 instead of char * is returned) of the function
to be coherent with other processors.

Signed-off-by: Stefano Babic <sbabic@denx.de>
CC: Detlev Zundel <dzu@denx.de>
CC: Fabio Estevam <fabio.estevam@freescale.com>