iio: adc: Split AD7091R-5 config symbol

Split AD7091R-5 kconfig symbol into one symbol for the base AD7091R driver
and another one for the I2C interface AD7091R-5 driver.

Signed-off-by: Marcelo Schmitt <marcelo.schmitt@analog.com>
Link: https://lore.kernel.org/r/8cae37c611c1b0fe3faef7a4b8c4cc915eaeddc7.1703013352.git.marcelo.schmitt1@gmail.com


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