iio: ad7266: Convert to use GPIO descriptors

The AD7266 have no in-tree users making use of the platform
data mechanism to pass address GPIO lines when not using
a fixed address, so we can easily convert this to use
GPIO descriptors instead of the platform data integers
currently passed.

Lowercase the labels "ad0".."ad2" as this will make a better
fit for platform descriptions like device tree that prefer
lowercase names such as "ad0-gpios" rather than "AD0-gpios".

Board files and other static users of this device can pass
the same GPIO descriptors using machine descriptor
tables if need be.

Cc: Alison Schofield <amsfield22@gmail.com>
Cc: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Reviewed-by: Alexandru Ardelean <alexandru.ardelean@analog.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>