xlnx-zynqmp-ipi: Initial version of the Xilinx IPI device

This is the initial version of the Inter Processor Interrupt device.

Signed-off-by: Alistair Francis <alistair.francis@xilinx.com>
Reviewed-by: Edgar E. Iglesias <edgar.iglesias@xilinx.com>
Signed-off-by: Edgar E. Iglesias <edgar.iglesias@xilinx.com>