power: supply: axp20x_usb_power: Use regmap fields for VBUS monitor feature

Use regmap fields to describe the VBUS valid bit and VBUS monitor
enable bit. This allows the driver to easily support other chips,
eg. the AXP192, that have the VBUS valid bit in a different register.

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