chore(docs): refresh platform ports landing page

 - Remove mentions of Arm SGM-775 and MediaTek MT6795 platforms.
   Both platform ports were deleted from TF-A source tree in the
   last release (v2.7).

 - Remove mention of Arm Morello platform, as it now has a dedicated
   documentation page accessible from the table of contents
   (see docs/plat/arm/morello/).

Change-Id: Ie3acdddab81f5589bb36114a8a766200f5b08ad4
Signed-off-by: Sandrine Bailleux <sandrine.bailleux@arm.com>
diff --git a/docs/plat/index.rst b/docs/plat/index.rst
index 28b1787..25d8771 100644
--- a/docs/plat/index.rst
+++ b/docs/plat/index.rst
@@ -59,10 +59,9 @@
    - Arm Neoverse N1 System Development Platform (N1SDP)
    - Arm Neoverse Reference Design N1 Edge (RD-N1-Edge) FVP
    - Arm Neoverse Reference Design E1 Edge (RD-E1-Edge) FVP
-   - Arm SGI-575 and SGM-775
-   - MediaTek MT6795 and MT8173 SoCs
-   - Arm Morello Platform
+   - Arm SGI-575
+   - MediaTek MT8173 SoCs
 
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-*Copyright (c) 2019-2020, Arm Limited. All rights reserved.*
+*Copyright (c) 2019-2022, Arm Limited. All rights reserved.*