power: supply: axp20x_usb_power: Simplify USB current limit handling

Handle the USB current limit with a lookup table and regmap field,
which minimizes code duplication. Invalid or unlimited values are
denoted by -1 entries, and can't be selected from userspace.

Signed-off-by: Aidan MacDonald <aidanmacdonald.0x0@gmail.com>
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>