MAINTAINERS: add include/dt-bindings/i2c to I2C SUBSYSTEM HOST DRIVERS

Maintainers of the directory Documentation/devicetree/bindings/i2c
are also the maintainers of the corresponding directory
include/dt-bindings/i2c.

Add the file entry for include/dt-bindings/i2c to the appropriate
section in MAINTAINERS.

Signed-off-by: Lukas Bulwahn <lukas.bulwahn@gmail.com>
Signed-off-by: Wolfram Sang <wsa@kernel.org>