max8903: adds support for initiation via device tree

Adds support for device tree to setup a max8903 battery charger. DC and USB
validity are determined by looking the presence of the dok and uok gpios.

Signed-off-by: Chris Lapa <chris@lapa.com.au>
Reviewed-by: Krzysztof Kozlowski <k.kozlowski@samsung.com>
Signed-off-by: Sebastian Reichel <sre@kernel.org>