power: supply: gpio-charger: Remove pdata from gpio_charger

Platform data are now used only during probe time, so remove
them from gpio_charger structure and consolidate probing
function accordingly.

Signed-off-by: Ladislav Michl <ladis@linux-mips.org>
[Replace of_property_read_string with dev_property_read_string]
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.co.uk>