arm64: zynqmp: Use C pre-processor for includes

Change the dtsi include code to use the C pre-processor #include instead
of the device tree /include/. This enables option to use dt binding
headers.

Signed-off-by: Alistair Francis <alistair.francis@xilinx.com>
Reviewed-by: Sören Brinkmann <soren.brinkmann@xilinx.com>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>