arm64: zynqmp: Make zynqmp_firmware driver optional

Make zynqmp_firmware driver as optional to disable it, if user don't
want to use default zynqmp firmware interface.

Signed-off-by: Tejas Patel <tejas.patel@xilinx.com>
Signed-off-by: Jolly Shah <jolly.shah@xilinx.com>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>