ARM i.MX6q: unmap memory mapped at imx6q_opp_check_speed_grading()

imx6q_opp_check_speed_grading() remaps memory to the base variable and
never unmaps it. I can't see how this can be of any use later so here I
unmap it.

Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Shawn Guo <shawn.guo@linaro.org>