imx: mx6: add get_cpu_speed_grade_hz func to return MHz speed grade from OTP

The IMX6 has four different speed grades determined by eFUSE SPEED_GRADING
indicated by OCOTP_CFG3[17:16] which is at 0x440 in the Fusemap Description
Table. Return this frequency so that it can be used elsewhere.

Note that the IMX6SDLRM and the IMX6SXRM do not indicate this in the
their Fusemap Description Table however Freescale has confirmed that these
eFUSE bits match the description within the IMX6DQRM and that they will
be added to the next revision of the respective reference manuals.

These have been tested with IMX6 Quad/Solo/Dual-light 800Mhz and 1GHz grades.

Signed-off-by: Tim Harvey <tharvey@gateworks.com>