build: deprecate Arm sgm775 FVP platform

sgm775 is an old platform and is no longer maintained by Arm and its
fast model FVP_CSS_SGM-775 is no longer available for download.
This platform is now superseded by Total Compute(tc) platforms.

This platform is now deprecated but the source will be kept for cooling
off period of 2 release cycle before removing it completely.
Signed-off-by: Manish Pandey <manish.pandey2@arm.com>
Change-Id: I8fe1fc3da0c508dba62ed4fc60cbc1642e0f7f2a

docs/plat/deprecated.rst

@@ -14,5 +14,5 @@ List of deprecated platforms
 +----------------+----------------+--------------------+--------------------+
 |    Platform    |     Vendor     | Deprecated version |  Deleted version   |
 +================+================+====================+====================+
-|                |                |                    |                    |
+|    sgm775      |      Arm       |        2.5         |       2.7          |
 +----------------+----------------+--------------------+--------------------+

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