dt-bindings: i2c: sh_mobile: Document R-Car M3-N support

Document support for the IIC Bus Interface for DVFS (IIC for DVFS) in
the Renesas M3-N (r8a77965) SoC.

No driver update is needed.

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