soc: xilinx: vcu: implement PLL disable

soc: xilinx: vcu: implement PLL disable

The disabling of the PLL is not fully implemented, because according to
the ZynqMP register reference the RESET, POR_IN and PWR_POR bits have to
be set to bring the PLL into reset.

Set the bits to disable the PLL.

Signed-off-by: Michael Tretter <m.tretter@pengutronix.de>
Acked-by: Michal Simek <michal.simek@xilinx.com>
Link: https://lore.kernel.org/r/20210121071659.1226489-7-m.tretter@pengutronix.de
Signed-off-by: Stephen Boyd <sboyd@kernel.org>