arm64: dts: zynqmp: Wire mailbox with zynqmp-power driver

The support to driver was added by commit ffdbae28 ("drivers: soc:
xilinx: Use mailbox IPI callback") that's why also enable it via DT by
default. It setups communication with firmware via IPI interface.

Signed-off-by: Michal Simek <michal.simek@xilinx.com>
Link: https://lore.kernel.org/r/5d3523150890e494df308ee69523d0f0e7b33b22.1605185549.git.michal.simek@xilinx.com