i2c: sh_mobile: document support for r8a7796 (R-Car M3-W)

Explicitly list per-SoC binding for r8a7796. No driver change
is required as the initialisation sequence is currently the same
as for the R-Car Gen3 fallback binding.

Signed-off-by: Simon Horman <horms+renesas@verge.net.au>
Acked-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Wolfram Sang <wsa@the-dreams.de>