iio: Add resource managed APIs devm_iio_channel_{get,release) in devres

Add following APIs in the list of managed resources of IIO:
	devm_iio_channel_get()
	devm_iio_channel_get_all()
	devm_iio_channel_release()
	devm_iio_channel_release_all()

Signed-off-by: Laxman Dewangan <ldewangan@nvidia.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>