power: supply: axp20x_battery: add support for DT battery

This adds support in X-Powers AXP20X and AXP22X battery driver for a
fixed battery in DT.

It will take the minimum supported voltage by the battery as defined in
the battery DT node and set the V_OFF register to this value, telling
the system to shut down if the supplied power is below this value.

Signed-off-by: Quentin Schulz <quentin.schulz@free-electrons.com>
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.co.uk>