Update to use generic function for displaying silicon revision

Tested on my mx53 loco board:
CPU identified as i.MX53, silicon rev 2.0

Test on my mx51 babbage board:
CPU identified as i.MX51, silicon rev 3.0
Signed-off-by: Jason Liu <jason.hui@linaro.org>
Cc: Fabio Estevam <fabio.estevam@freescale.com>
Cc: Sascha Hauer <s.hauer@pengutronix.de>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>