doc: renesas: Document platforms based on RZ/G2 SoC's

Document the platforms based on RZ/G2 SoC's.

Signed-off-by: Biju Das <biju.das.jz@bp.renesas.com>
Reviewed-by: Lad Prabhakar <prabhakar.mahadev-lad.rj@bp.renesas.com>
Change-Id: I9ce5b9df3573b1198c5c7be79b5471d54573609a
diff --git a/docs/plat/index.rst b/docs/plat/index.rst
index fb60e56..3cbb552 100644
--- a/docs/plat/index.rst
+++ b/docs/plat/index.rst
@@ -32,6 +32,7 @@
    rpi3
    rpi4
    rcar-gen3
+   rz-g2
    rockchip
    socionext-uniphier
    synquacer