docs: introduce process for platform deprecation

Signed-off-by: Manish Pandey <manish.pandey2@arm.com>
Change-Id: Ifb8a3220f2fc2286fa91614887d17f54178ed002

docs/plat/deprecated.rst

+Deprecated platforms
+====================
+
+Process of deprecating a platform
+---------------------------------
+
+Platform can be deprecated and its source can be kept in repository for a cooling
+off period before deleting it or it can be deleted straight away. For later types
+Deprecated/Deleted version would be same.
+
+List of deprecated platforms
+----------------------------
+
++----------------+----------------+--------------------+--------------------+
+|    Platform    |     Vendor     | Deprecated version |  Deleted version   |
++================+================+====================+====================+
+|                |                |                    |                    |
++----------------+----------------+--------------------+--------------------+

docs/plat/index.rst

@@ -9,6 +9,7 @@ Platform Ports
 
    allwinner
    arm/index
+   deprecated
    meson-axg
    meson-gxbb
    meson-gxl