zynqmp: pm: Add support for setting PMU configuration object

Allow EL2 (e.g. U-Boot) to load the configuration object at runtime
into the Xilinx ZynqMP PMU firmware. This allows booting with U-Boot
and U-Boot SPL with PMU FW without hard-coding the configuration
object.

Signed-off-by: Luca Ceresoli <luca@lucaceresoli.net>