iio:ak8975 Add support for gpios DT property

Add support for parsing 'gpios' property when initializing
from oftree.
This patch adds also the binding documentation file.

Signed-off-by: Jacek Anaszewski <j.anaszewski@samsung.com>
Signed-off-by: Kyungmin Park <kyungmin.park@samsung.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>