iio: adc: at91-sama5d2_adc: remove unused definition

Remove unused register definition

Signed-off-by: Eugen Hristev <eugen.hristev@microchip.com>
Link: https://lore.kernel.org/r/20210901123013.329792-4-eugen.hristev@microchip.com


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