ARM: dts: imx6q: add 852MHz setpoint for CPU freq

According to datasheet, i.MX6Q has setpoint of 852MHz
which is exclusive with 996MHz, the fuse map of speed_grading
defines the max speed of ARM, here we add this 852MHz
setpoint opp info, kernel will check the speed_grading
fuse and remove all illegal setpoints.

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