docs: introduce process for platform deprecation

Signed-off-by: Manish Pandey <manish.pandey2@arm.com>
Change-Id: Ifb8a3220f2fc2286fa91614887d17f54178ed002
diff --git a/docs/plat/index.rst b/docs/plat/index.rst
index 3cbb552..e8815dd 100644
--- a/docs/plat/index.rst
+++ b/docs/plat/index.rst
@@ -9,6 +9,7 @@
 
    allwinner
    arm/index
+   deprecated
    meson-axg
    meson-gxbb
    meson-gxl