docs(imx9): add imx93 platform

Add i.MX9 platform introduction.

Signed-off-by: Jacky Bai <ping.bai@nxp.com>
Change-Id: Ic07ca394cff6a9e3e21b7a03f9c9080d3c1ef91a
diff --git a/docs/plat/index.rst b/docs/plat/index.rst
index 188c986..fe2cc44 100644
--- a/docs/plat/index.rst
+++ b/docs/plat/index.rst
@@ -27,6 +27,7 @@
    warp7
    imx8
    imx8m
+   imx9
    nxp/index
    poplar
    qemu