power: pm8921-charger: add dc_unplug_check option

Add a configuration option to enable or disable
the reverse boost fix for the DC charging path.

Some board designs do not utilize the DC_IN path
on the PMIC, however they do use the presence
detection of DC_IN to enable the auto switching
and interrupt features. In these particular configurations
the PMIC does not control the DC OVP FETs.
Hence add an option to enable or disable the DC path workaround
and leave the workaround disabled by default.

Change-Id: I1e55bbfe37c7cc6f62581af11280a565ed648c06
Signed-off-by: David Keitel <dkeitel@codeaurora.org>