power: supply: axp20x_usb_power: Use a match structure

Instead of ad-hoc variant ID checks throughout the code, let's start
moving the variant-specific details to a match structure. This allows
for future flexibility, and it better matches the other axp20x power
supply drivers.

This commit removes most variant checks from axp20x_usb_power_probe().
Other parts of the driver still do ID matching; they are left unchanged
for now.

Reviewed-by: Chen-Yu Tsai <wens@csie.org>
Signed-off-by: Samuel Holland <samuel@sholland.org>
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>