According to latest docs imx7d chips can go from 800 to 1200 mhz.
Maximum frequency is determined from two speed grading bits present in
OCOTP fuses at same location as other imx chips.

Also update to "typical" voltages from latest datasheet, 25mv higher
than current dts.

All imx7s parts are still fixed at 800mhz

Based on:
* IMX7DCEC Rev. 6, 03/2019
* IMX7SCEC Rev. 6, 03/2019
* IMX7DRM Rev. 1, 01/2018 Page 1102
Signed-off-by: Leonard Crestez <leonard.crestez@nxp.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>