docs(imx8ulp): add imx8ulp platform

Add i.MX8ULP platform introduction.

Signed-off-by: Jacky Bai <ping.bai@nxp.com>
Change-Id: Idc16bcf5b23542f8a1f394a474309239ddcb9685
diff --git a/docs/plat/index.rst b/docs/plat/index.rst
index b1ccaa5..1069635 100644
--- a/docs/plat/index.rst
+++ b/docs/plat/index.rst
@@ -27,6 +27,7 @@
    warp7
    imx8
    imx8m
+   imx8ulp
    imx9
    npcm845x
    nxp/index