ARM: imx: Remove unused chip revision strings

Since commit (167a19d2: ARM: imx: Introduce generic function for displaying silicon revision)
we no longer need the chip revision strings, so remove them.

Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>