MAINTAINERS: Add MAINTAINERS entry for AD7091R

The driver for AD7091R was added in
ca693001: iio: adc: Add support for AD7091R5 ADC
but no MAINTAINERS file entry was added for it since then.
Add a proper MAINTAINERS file entry for the AD7091R driver.

Signed-off-by: Marcelo Schmitt <marcelo.schmitt@analog.com>
Link: https://lore.kernel.org/r/4247e653354f8eb362264189db24c612d5e4e131.1703013352.git.marcelo.schmitt1@gmail.com


Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>