dt-bindings: arm: renesas: Document beacon-rzg2m

Beacon EmbeddedWorks is introducing a development kit based on the
Renesas RZ/G2M platform.  This patch adds the entry to the bindings
list.

Signed-off-by: Adam Ford <aford173@gmail.com>
Link: https://lore.kernel.org/r/20200714123419.3390-2-aford173@gmail.com


Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>