Add "Standard", "Adaptive", and "Custom" modes to the charge_type
property, to expand the existing "Trickle" and "Fast" modes.
I am adding them in order to support a new Chrome OS device,
but these properties should be general enough that they can be
used on other devices.

The meaning of "Standard" is obvious, but "Adaptive" and "Custom" are
more tricky: "Adaptive" means that the charge controller uses some
custom algorithm to change the charge type automatically, with no
configuration needed. "Custom" means that the charge controller uses the
POWER_SUPPLY_PROP_CHARGE_CONTROL_* properties as configuration for some
other algorithm.

v5 changes:
- Split up adding the charge types and adding the
  POWER_SUPPLY_PROP_CHARGE_CONTROL_START_THRESHOLD and
  POWER_SUPPLY_PROP_CHARGE_CONTROL_END_THRESHOLD properties into
  two different commits.
v4 changes:
- Add documentation for the new properties, and add documentation for
  the the previously missing charge_control_limit and
  charge_control_limit_max properties.
Signed-off-by: Nick Crews <ncrews@chromium.org>
Reviewed-by: Enric Balletbo i Serra <enric.balletbo@collabora.com>
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>