power: supply: axp20x_usb_power: use IIO channels when available

power: supply: axp20x_usb_power: use IIO channels when available

The X-Powers AXP20X PMIC exposes the current current and voltage
measures via an internal ADC.

This adds the possibility to read IIO channels directly for processed
values rather than reading the registers and computing the value.

For backward compatibility purpose, if the IIO driver is not compiled,
this driver will fall back on previous behaviour which is direct
register readings.

Signed-off-by: Quentin Schulz <quentin.schulz@free-electrons.com>
Acked-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Acked-by: Jonathan Cameron <jic23@kernel.org>
Signed-off-by: Sebastian Reichel <sre@kernel.org>