iio: adc: Move AD7091R5 entry in a alphabetical order in Makefile

Ad7091R5 was added in a non alphabetical order after AD7124 in Makefile and
KConfig. This patch fixes that and place Ad7091R5 before AD7124.

Signed-off-by: Beniamin Bia <beniamin.bia@analog.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>