ARM: imx: Introduce generic function for displaying silicon revision

Currently each i.MX processor has its own function for displaying the silicon
revision.

Introduce a generic function for this purpose, so that all i.MX processors can
reuse it.

Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>
Signed-off-by: Jason Liu <jason.hui@linaro.org>
Cc: Sascha Hauer <s.hauer@pengutronix.de>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>