iio: Add support for DA9150 GPADC

This patch adds support for DA9150 Charger & Fuel-Gauge IC GPADC.

Signed-off-by: Adam Thomson <Adam.Thomson.Opensource@diasemi.com>
Reviewed-by: Hartmut Knaack <knaack.h@gmx.de>
Acked-by: Jonathan Cameron <jic23@kernel.org>
Signed-off-by: Sebastian Reichel <sre@kernel.org>