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

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>