dt-bindings: arm: Document RZ/G2E SoC DT bindings

Add device tree bindings documentation for Renesas RZ/G2E (r8a774c0)
SoC.

Signed-off-by: Fabrizio Castro <fabrizio.castro@bp.renesas.com>
Reviewed-by: Biju Das <biju.das@bp.renesas.com>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>