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>

docs/plat/index.rst

@@ -9,6 +9,7 @@ Platform Ports
    allwinner
    meson-gxbb
    meson-gxl
+   meson-g12a
    fvp_ve
    hikey
    hikey960
@@ -23,7 +24,9 @@ Platform Ports
    ls1043a
    poplar
    qemu
+   qemu-sbsa
    rpi3
+   rpi4
    rcar-gen3
    rockchip
    socionext-uniphier

docs/plat/qemu_sbsa.rst → docs/plat/qemu-sbsa.rst

-Trusted Firmware-A for QEMU SBSA target
-========================================
+QEMU SBSA Target
+================
 
 Trusted Firmware-A (TF-A) implements the EL3 firmware layer for QEMU SBSA
 Armv8-A. While running Qemu from command line, we need to supply two Flash