ARM: imx6q: print silicon version on boot

i.MX6Q has 3 revisions 1.0, 1.1 and 1.2.  Print revision on boot.

Signed-off-by: Drew Moseley <drew_moseley@mentor.com>
[drew: add proper error checking for function imx6q_revision()]
Signed-off-by: Shawn Guo <shawn.guo@linaro.org>