power: regulator: Add a driver for the AXP USB power supply

This driver reports the presence/absence of voltage on the PMIC's USB
VBUS pin. This information is used by the USB PHY driver. The
corresponding Linux driver uses the power supply class, which does not
exist in U-Boot. UCLASS_REGULATOR seems to be the closest match.

Signed-off-by: Samuel Holland <samuel@sholland.org>
Acked-by: Andre Przywara <andre.przywara@arm.com>
Signed-off-by: Andre Przywara <andre.przywara@arm.com>