power: qpnp-charger: make CHARGING_ENABLED property writable

Make the power supply framework property
POWER_SUPPLY_PROPERTY_CHARGING_ENABLED writeable in its sysfs node.

This removes the dependency on module parameters to disable and re-enable
charging.

Disable charging using the following command:

 echo 0 > /sys/class/power_supply/battery/charging_enabled

Change-Id: I4518f6e9079229d68f9cfb8e520bd2a23ee6a7da
Signed-off-by: David Keitel <dkeitel@codeaurora.org>