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>