docs: nxp soc-lx2160a based platforms

Addition of documents for platforms based on
NXP SoC LX2160A.

Signed-off-by: Pankaj Gupta <pankaj.gupta@nxp.com>
Change-Id: I39ac5a9eb0b668d26301a0a24a1e6bf87f245f02
diff --git a/docs/plat/nxp/index.rst b/docs/plat/nxp/index.rst
new file mode 100644
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+NXP Reference Development Platforms
+===================================
+
+.. toctree::
+   :maxdepth: 1
+   :caption: Contents
+
+   nxp-layerscape
+   nxp-ls-fuse-prov
+   nxp-ls-tbbr
+
+This chapter holds documentation related to NXP reference development platforms.
+It includes details on image flashing, fuse provisioning and trusted board boot-up.
+
+--------------
+
+*Copyright (c) 2021, NXP Limited. All rights reserved.*