ARM: mx51: Print silicon revision on boot

Having the silicon revision to appear on the boot log is a useful information.

MX31 and MX35 already show the silicon revision on boot.

Add support for displaying such information for MX51 as well.

Tested on a MX51EVK, where it shows:

CPU identified as i.MX51, silicon rev 3.0

Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>