ARM: imx: add sleep for pllv3 relock

The pllv3 relock time varies in the range of 50us ~ 500us, depending on
the specific PLL type, e.g. 50us for ARM PLL and 450us for Audio/Video
PLL.  Let's add a usleep_range() call instead of doing busy wait during
relock.

Signed-off-by: Shawn Guo <shawn.guo@linaro.org>