iio: adc: at91-sama5d2_adc: add support for separate end of conversion registers

iio: adc: at91-sama5d2_adc: add support for separate end of conversion registers

Some platforms have separated the end-of-conversion information from the
usual ISR/IMR/IER/IDR registers, into EOC_ISR/EOC_IMR/EOC_IER/EOC_IDR.
To cope with both variants, helpers are being added, that will make
code more clear and more easy to read.

Signed-off-by: Eugen Hristev <eugen.hristev@microchip.com>
Link: https://lore.kernel.org/r/20210901123013.329792-6-eugen.hristev@microchip.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>