Merge changes from topic "sgm775_deprecation" into integration

* changes:
  build: deprecate Arm sgm775 FVP platform
  docs: introduce process for platform deprecation

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   |
++================+================+====================+====================+
+|    sgm775      |      Arm       |        2.5         |       2.7          |
++----------------+----------------+--------------------+--------------------+

docs/plat/index.rst

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

plat/arm/board/sgm775/platform.mk

@@ -4,6 +4,8 @@
 # SPDX-License-Identifier: BSD-3-Clause
 #
 
+$(warning Platform ${PLAT} is deprecated. Some of the features might not work as expected)
+
 include plat/arm/css/sgm/sgm-common.mk
 
 SGM775_BASE= plat/arm/board/sgm775