dt-bindings: i2c: sh_mobile: Document r8a7743/5 support

Document i2c Device Tree support for RZ/G1[ME]
(also known as r8a774[35]) SoCs. They are compatible with
R-Car Gen2 SoC family. No driver change is required as the
initialisation sequence is currently the same as for the
R-Car Gen2 fallback binding.

Signed-off-by: Biju Das <biju.das@bp.renesas.com>
Acked-by: Simon Horman <horms+renesas@verge.net.au>
Signed-off-by: Wolfram Sang <wsa@the-dreams.de>