power: power_supply_core: Add API to enable and disable charging

To allow external drivers enable and disable charging add
a power supply framework property POWER_SUPPLY_PROP_CHARGING_ENABLED
which reflects the status of charging being enabled or disabled.

Change-Id: I199a08797b8f2f00edee05e83735c4835400aa60
Signed-off-by: David Keitel <dkeitel@codeaurora.org>