imx: mx6: add get_cpu_temp_grade to obtain cpu temperature grade from OTP

The MX6 has a temperature grade defined by OCOTP_MEM0[7:6] which is at 0x480
in the Fusemap Description Table in the reference manual. Return this value
as well as min/max temperature based on the value.

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.

This has been tested with IMX6 Automative and Industrial parts.

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