firmware: xilinx: Add Zynqmp firmware driver

This patch is adding communication layer with firmware.
Firmware driver provides an interface to firmware APIs.
Interface APIs can be used by any driver to communicate to
PMUFW(Platform Management Unit). All requests go through ATF.

Signed-off-by: Rajan Vaja <rajanv@xilinx.com>
Signed-off-by: Jolly Shah <jollys@xilinx.com>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>