MAINTAINERS: adc: at91-sama5d2_adc: remove myself as co-maintainer

Eugen is, now, more active and up to date on this topic than I. So I let
him the full maintainance of this driver.

Signed-off-by: Ludovic Desroches <ludovic.desroches@microchip.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>