Update the mx25_read_cpu_rev function to recognize silicon revision 1.2
for imx25 chipsets.

Silicon revision 1.2 is mentioned in the errata document at
https://www.nxp.com/docs/en/errata/IMX25CE.pdf. The imx25 chips on my
test boards show revision 1.2 as well.
Signed-off-by: Martin Kaiser <martin@kaiser.cx>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>