ARM: mx6: Only check for 1.2GHz for mx6quad

It is only the mx6quad variant that can run up to 1.2GHz, so add the check
accordingly.

This avoids getting the following warning on a mx6solo:

failed to disable 1.2 GHz OPP

Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>
Signed-off-by: Shawn Guo <shawn.guo@freescale.com>