power: supply: core: Add CHARGE_CONTROL_{START_THRESHOLD,END_THRESHOLD} properties

Add POWER_SUPPLY_PROP_CHARGE_CONTROL_START_THRESHOLD
and POWER_SUPPLY_PROP_CHARGE_CONTROL_END_THRESHOLD properties, to expand
the existing CHARGE_CONTROL_* properties. 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.

When the charge_type is "Custom", the charge controller uses the
POWER_SUPPLY_PROP_CHARGE_CONTROL_* properties as configuration for some
other algorithm. For example, in the use case that I am supporting,
this means the battery begins charging when the percentage
level drops below POWER_SUPPLY_PROP_CHARGE_CONTROL_START_THRESHOLD and
charging ceases when the percentage level goes above
POWER_SUPPLY_PROP_CHARGE_CONTROL_END_THRESHOLD.

v5 changes:
- Add the other missing CHARGE_CONTROL_* properties documentation in
  a separate commit
- 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>