ARM: mx31: Print the silicon version

Use the same method of the Linux kernel to print the MX31 silicon version on
boot.

Tested on a MX31PDK with a 2.0 silicon, where it shows:

CPU:   Freescale i.MX31 rev 2.0 at 531 MHz

Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>