soc: xilinx: add a to_zynqmp_pm_domain macro

Replace container_of for converting a generic_pm_domain to a
zynqmp_pm_domain with a macro definition to simplify the code.

Signed-off-by: Michael Tretter <m.tretter@pengutronix.de>
Acked-by: Michal Simek <michal.simek@xilinx.com>
Acked-by: Rajan Vaja <rajan.vaja@xilinx.com>
Link: https://lore.kernel.org/r/20210825150313.4033156-5-m.tretter@pengutronix.de


Signed-off-by: Michal Simek <michal.simek@xilinx.com>