The X-Powers AXP813 PMIC has got some slight differences from
AXP20X/AXP22X PMICs:
 - the maximum voltage supplied by the PMIC is 4.35 instead of 4.36/4.24
 for AXP20X/AXP22X,
 - the constant charge current formula is different,

It also has a bit to tell whether the battery percentage returned by the
PMIC is valid.
Signed-off-by: Quentin Schulz <quentin.schulz@bootlin.com>
Reviewed-by: Chen-Yu Tsai <wens@csie.org>
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.co.uk>