power: supply: add battery driver for AXP20X and AXP22X PMICs

The X-Powers AXP20X and AXP22X PMICs can have a battery as power supply.

This patch adds the battery power supply driver to get various data from
the PMIC, such as the battery status (charging, discharging, full,
dead), current max limit, current current, battery capacity (in
percentage), voltage max and min limits, current voltage and battery
capacity (in Ah).

This battery driver uses the AXP20X/AXP22X ADC driver as PMIC data
provider.

Signed-off-by: Quentin Schulz <quentin.schulz@free-electrons.com>
Acked-by: Jonathan Cameron <jic23@kernel.org>
Acked-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Acked-by: Chen-Yu Tsai <wens@csie.org>
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.co.uk>