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

Rename the bindings documentation file for R-Car I2C controller
from i2c-rcar.txt to renesas,i2c.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 <[e-mail]>
Reviewed-by: Geert Uytterhoeven <[e-mail]>
Signed-off-by: Wolfram Sang <[e-mail]>

MAINTAINERS

···
 RENESAS R-CAR I2C DRIVERS
 M:	Wolfram Sang <[e-mail]>
 S:	Supported
-F:	Documentation/devicetree/bindings/i2c/i2c-rcar.txt
+F:	Documentation/devicetree/bindings/i2c/renesas,i2c.txt
 F:	Documentation/devicetree/bindings/i2c/renesas,iic.txt
 F:	drivers/i2c/busses/i2c-rcar.c
 F:	drivers/i2c/busses/i2c-sh_mobile.c