ARM: imx: add speed grading check for i.mx6 soc

The fuse map of speed_grading[1:0] defines the max speed
of ARM, see below the definition:

2b'11: 1200000000Hz;
2b'10: 996000000Hz;
2b'01: 852000000Hz; -- i.MX6Q Only, exclusive with 996MHz.
2b'00: 792000000Hz;

Need to remove all illegal setpoints according to fuse
map.

Signed-off-by: Anson Huang <b20788@freescale.com>
Signed-off-by: Shawn Guo <shawn.guo@linaro.org>