soc: xilinx: vcu: add helper to wait for PLL locked

soc: xilinx: vcu: add helper to wait for PLL locked

Extract a helper function to wait until the PLL is locked. Also,
disabling the bypass was buried in the exit path on the wait loop.
Separate the different steps and add a helper function to make the code
more readable.

Signed-off-by: Michael Tretter <m.tretter@pengutronix.de>
Acked-by: Michal Simek <michal.simek@xilinx.com>
Link: https://lore.kernel.org/r/20210121071659.1226489-5-m.tretter@pengutronix.de
Signed-off-by: Stephen Boyd <sboyd@kernel.org>