ARM: mx31: use generic function for displaying silicon revision

Update to use generic function for displaying silicon revision

Tested on mx31pdk board as print the following information:
CPU identified as i.MX31, silicon rev 2.0

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>