arm64: zynqmp: Add firmware DT node

Add firmware DT node in ZynqMP device tree. This node
uses bindings as per new firmware interface driver.

Signed-off-by: Rajan Vaja <rajan.vaja@xilinx.com>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>
Signed-off-by: Michael Tretter <m.tretter@pengutronix.de>