MAINTAINERS: Fix the link to ad7606 dt-bindings

The devicetree bindings documentation for ad7606 should also include
the vendor prefix: ad7606.txt -> adi,ad7606.txt

Fixes: 6e33a125 ("dt-bindings: iio: adc: Add docs for AD7606 ADC")
Signed-off-by: Stefan Popa <stefan.popa@analog.com>
Reviewed-by: Mukesh Ojha <mojha@codeaurora.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>