docs(zynqmp): add zynqmp documentation

Add information about ZynqMP platform for TF-A Tests and provide
the build commands.

Change-Id: I508d062d2ce16201b578e95d14d70e137ad709bf
Signed-off-by: Amit Nagal <amit.nagal@amd.com>
diff --git a/docs/plat/index.rst b/docs/plat/index.rst
index 6f5d9a5..33c0ab9 100644
--- a/docs/plat/index.rst
+++ b/docs/plat/index.rst
@@ -8,6 +8,7 @@
 
    xilinx-versal_net
    xilinx-versal
+   xilinx-zynqmp
 
 This section provides a list of supported upstream *platform ports* and the
 documentation associated with them.