Instead of reading the silicon version from ROM, we should
read the SREV register from the IIM.

Freescale has dropped all support for MX51 REV1.0, only MX51
REV 2.0 and 3.0 are valid.
Signed-off-by: Dinh Nguyen <Dinh.Nguyen@freescale.com>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>