dt-bindings: power: supply: Add battery.txt with simple-battery binding

Documentation of static battery characteristics that can be defined
for batteries that do not embed this data, which are required by
fuel-gauge and charger chips for proper handling of the battery.

The following properties are defined:
  voltage-min-design-microvolt
  charge-full-design-microamp-hours
  energy-full-design-microwatt-hours
  precharge-current-microamp
  charge-term-current-microamp
  constant-charge-current-max-microamp
  constant-charge-voltage-max-microamp

Property names are derived from corresponding elements in
enum power_supply_property from include/linux/power_supply.h
https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/include/linux/power_supply.h



Cc: Rob Herring <robh@kernel.org>
Cc: devicetree@vger.kernel.org
Signed-off-by: Matt Ranostay <matt@ranostay.consulting>
Signed-off-by: Liam Breck <kernel@networkimprov.net>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.co.uk>