doc: Add more missing platforms

Add meson-g12a, qemu-sbsa and rpi4 to the documentation index so
that they will have their docs rendered and integrated into the
table of contents.

Change-Id: Id972bf2fee67312dd7bff29f92bea67842e62431
Signed-off-by: Paul Beesley <paul.beesley@arm.com>
2 files changed