MAINTAINERS: iio: add co-maintainer to SAMA5D2-compatible ADC driver

Add Eugen as co-maintainer with Ludovic of Microchip SAMA5D2-compatible
ADC driver.
Also add the binding documentation/include as file pattern.

Signed-off-by: Nicolas Ferre <nicolas.ferre@microchip.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>