power: qpnp-charger: add optional charging-disabled flag

Add the chg-charging-disabled flag to allow the charger
to be disabled by default through device tree configuration.

This is required for some factory tests and RF measurements
which require no current to be drawn from the charger.

Change-Id: Ibdbdd5f62e0205ee12e53e0b918914769e015f13
Signed-off-by: David Keitel <dkeitel@codeaurora.org>