ARM: imx: pllv3 needs relock in .set_rate() call

The pllv3 nees relock not only when powering up but also when rate
changes.  The patch creates a helper function clk_pllv3_wait_lock() and
moves the relock code from clk_pllv3_prepare() into there, so that
both .prepare() and .set_rate() hooks of pllv3 can call into the helper
for relocking.

Since relock is only needed when PLL is powered up while clk_set_rate()
could be called before clk is prepared, we need to add a check in
clk_pllv3_wait_lock() to skip the relock if PLL is not powered.

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