Xilinx Zynq MPSoC Device Tree Bindings

Zynq MPSoC based systems rely on the "psci" node to detect the presence of PMU
firmware as well as to determine the calling method (either "smc" or "hvc")
to the PM-API firmware layer.

Required properties:
 - compatible  : Must contain:  "xlnx,zynqmp-pm"

 - method      : The method of calling the PM-API firmware layer.
                 Permitted values are:
                 "smc" : To be used in configurations without a hypervisor
                 "hvc" : To be used when hypervisor is present
