MAINTAINERS: update atmel,sama5d2-adc.yaml reference

Changeset 58ff1b51 ("dt-bindings:iio:adc:atmel,sama5d2-adc: txt to yaml conversion")
renamed: Documentation/devicetree/bindings/iio/adc/at91-sama5d2_adc.txt
to: Documentation/devicetree/bindings/iio/adc/atmel,sama5d2-adc.yaml.

Update its cross-reference accordingly.

Fixes: 58ff1b51 ("dt-bindings:iio:adc:atmel,sama5d2-adc: txt to yaml conversion")
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>