arm64: dts: zynqmp: Add label for zynqmp_ipi

Add label which is used by bootloader for adding bootloader specific flag.

Signed-off-by: Michal Simek <michal.simek@xilinx.com>
Link: https://lore.kernel.org/r/3dc8416abdd3498e61edcd83830a12af295c5c6d.1611224800.git.michal.simek@xilinx.com