iio: adc: at91-sama5d2_adc: add helper for COR register

iio: adc: at91-sama5d2_adc: add helper for COR register

Add helper for the COR register. This helper allows to modify the COR
register, removes duplicate code and improves readability.
The COR offset is now part of the register layout. This will allow
different platform with a different offset to use the same helper.

Signed-off-by: Eugen Hristev <eugen.hristev@microchip.com>
Link: https://lore.kernel.org/r/20210901123013.329792-7-eugen.hristev@microchip.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>