xen/zynqmp: add IPI calls virtualization

xen/zynqmp: add IPI calls virtualization

ZynqMP IPI mailbox calls are a small set of EEMI sister calls, often
used in conjunction with EEMI related functionalities.

Unfortunately they are not part of the EEMI spec, or any other public
spec, but the implementation is upstream in ATF:

https://github.com/ARM-software/arm-trusted-firmware/blob/master/plat/xilinx/zynqmp/ipi_mailbox_service/ipi_mailbox_svc.h

And patches are close to getting into Linux:

https://patchwork.kernel.org/cover/10689501/

Signed-off-by: Stefano Stabellini <stefanos@xilinx.com>
Acked-by: Julien Grall <julien.grall@arm.com>