i2c: sh_mobile: Document SoC-specific bindings

Explicitly list the various SoC-specific compatible properties.
This allows checkpatch to validate DTSes.

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