xlnx-zynqmp-pmu: Initial commit of the ZynqMP PMU

The Xilinx ZynqMP SoC has two main processing systems in it. The ARM
processing system (which is already modeled in QEMU) and the MicroBlaze
Power Management Unit (PMU). This is the inital work for adding support
for the PMU.

The PMU susbsystem runs along side the ARM system on hardware, but due
to architecture limitations in QEMU the two instances are seperate for
the time being.

Let's follow the same setup we do with the ARM system, where there is an
SoC device and a ZCU102 board. Although the PMU is less board specific
we are still going to follow the same split as maybe in future we can
connect the PMU device to the ARM ZCU102 board. As the machine will be
fairly small let's keep them both together in one file.

Signed-off-by: Alistair Francis <alistair.francis@xilinx.com>
Reviewed-by: Edgar E. Iglesias <edgar.iglesias@xilinx.com>
Signed-off-by: Edgar E. Iglesias <edgar.iglesias@xilinx.com>