power: supply: axp20x_usb_power: set min voltage and max current from sysfs

AXP20X and AXP22X PMICs allow setting the min voltage and max current of
VBUS power supply. This adds entries in sysfs to allow to do so.

Signed-off-by: Quentin Schulz <quentin.schulz@free-electrons.com>
Acked-by: Chen-Yu Tsai <wens@csie.org>
Signed-off-by: Sebastian Reichel <sre@kernel.org>