Add IPI mailbox service to manage Xilinx ZynqMP IPI(Inter Processors
Interrupt) access.

Signed-off-by: Wendy Liang <jliang@xilinx.com>

To make software license auditing simpler, use SPDX[0] license
identifiers instead of duplicating the license text in every file.

NOTE: Files that have been imported by FreeBSD have not been modified.

[0]: https://spdx.org/

Change-Id: I80a00e1f641b8cc075ca5a95b10607ed9ed8761a
Signed-off-by: dp-arm <dimitris.papastamos@arm.com>

The Xilinx Zynq UltraScale+ MPSOC containes a quad A53 cluster. This
patch adds the platform port for that SoC.

Signed-off-by: Soren Brinkmann <soren.brinkmann@xilinx.com>