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/index.rst b/docs/plat/index.rst
index 4dc9ecd..5848005 100644
--- a/docs/plat/index.rst
+++ b/docs/plat/index.rst
@@ -27,6 +27,7 @@
    imx8
    imx8m
    ls1043a
+   nxp/index
    poplar
    qemu
    qemu-sbsa