dt-bindings: i2c: sh_mobile: Rename bindings documentation file

Rename the bindings documentation file for sh_mobile I2C controller
from i2c-sh_mobile.txt to renesas,iic.txt.

This is part of an ongoing effort to name bindings documentation files for
Renesas IP blocks consistently, in line with the compat strings they
document.

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