soc: xilinx: vcu: make pll post divider explicit

soc: xilinx: vcu: make pll post divider explicit

According to the downstream driver documentation due to timing
constraints the output divider of the PLL has to be set to 1/2. Add a
helper function for that check instead of burying the code in one large
setup function.

The bit is undocumented and marked as reserved in the register
reference.

Signed-off-by: Michael Tretter <m.tretter@pengutronix.de>
Acked-by: Michal Simek <michal.simek@xilinx.com>
Link: https://lore.kernel.org/r/20210121071659.1226489-10-m.tretter@pengutronix.de
Signed-off-by: Stephen Boyd <sboyd@kernel.org>