iio: frequency: adf4350: Add support for dt bindings

Per review feedback from Lars-Peter Clausen <lars@metafoo.de>
Changes since V1:
	Fix return value handling of adf4350_parse_dt()
	Use of_get_gpio
	Avoid abbreviations in devicetree properties
	Fix typo in docs

Signed-off-by: Michael Hennerich <michael.hennerich@analog.com>
Reviewed-by: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>