doc: board: xilinx: Add documentation for ZynqMP

Add missing documentation for Xilinx ZynqMP configuration with steps how
to configure it and run it.

Signed-off-by: Michal Simek <michal.simek@xilinx.com>
diff --git a/doc/board/xilinx/index.rst b/doc/board/xilinx/index.rst
index 47f0929..e9560f2 100644
--- a/doc/board/xilinx/index.rst
+++ b/doc/board/xilinx/index.rst
@@ -8,3 +8,4 @@
 
    xilinx
    zynq
+   zynqmp