dt-bindings: arm: renesas: Add HopeRun RZ/G2[M] boards

This patch adds board HiHope RZ/G2M (the main board, powered by
the R8A774A1) and board HiHope RZ/G2 EX (the expansion board
that sits on top of the HiHope RZ/G2M). Both boards are made
by Jiangsu HopeRun Software Co., Ltd. (a.k.a. HopeRun).

Useful links:
http://hihope.org/product/detail/rzg2
https://item.taobao.com/item.htm?spm=a2oq0.12575281.0.0.6bcf1debQpzkRS&ft=t&id=592177498472
http://www.hoperun.com/Cn/news/id/379

We already know that the HiHope RZ/G2 EX will also sit on the
HiHope RZ/G2N, even though the HiHope RZ/G2N doesn't exist just
yet.

Signed-off-by: Fabrizio Castro <fabrizio.castro@bp.renesas.com>
Reviewed-by: Chris Paterson <Chris.Paterson2@renesas.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>

Documentation/devicetree/bindings/arm/renesas.yaml

@@ -106,6 +106,14 @@ properties:
 
       - description: RZ/G2M (R8A774A1)
         items:
+          - enum:
+              - hoperun,hihope-rzg2m # HopeRun HiHope RZ/G2M platform
+          - const: renesas,r8a774a1
+
+      - items:
+          - enum:
+              - hoperun,hihope-rzg2-ex # HopeRun expansion board for HiHope RZ/G2 platforms
+          - const: hoperun,hihope-rzg2m
           - const: renesas,r8a774a1
 
       - description: RZ/G2E (R8A774C0)