Rather than doing this in the SoC specific code just print
it in imx_set_silicon_revision. This saves some lines of code
and also results in i.MX27 now also having the silicon revision
printed during startup.

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>

All i.MX SoCs now use the same imx_silicon_revision() function to get
the revision. Add a separate header file for it and a common function
used on all SoCs.

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>