MAINTAINERS: add include/dt-bindings/iio to IIO SUBSYSTEM AND DRIVERS

MAINTAINERS: add include/dt-bindings/iio to IIO SUBSYSTEM AND DRIVERS

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

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

Signed-off-by: Lukas Bulwahn <lukas.bulwahn@gmail.com>
Link: https://lore.kernel.org/r/20220613115045.24326-1-lukas.bulwahn@gmail.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>