power: supply: axp20x_battery: allow disabling battery charging

Allow disabling and re-enabling battery charging of an axp209 PMIC
through a writable status property. With the current driver code
charging is always on.

This works on the axp209 of Banana {Pi M1+,Pro} and should work on all
AXP chips.

Signed-off-by:  <Hermann.Lauer@uni-heidelberg.de>
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>