power: supply: sc27xx: Add CURRENT_NOW/VOLTAGE_NOW properties support

Add new properties to get present current and voltage of the fuel gauge.

Signed-off-by: Yuanjiang Yu <yuanjiang.yu@unisoc.com>
Signed-off-by: Baolin Wang <baolin.wang7@gmail.com>
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>