power: qpnp-linear-charger: Add collapsible charger support

VIN_MIN comparator is used to prevent USBIN voltage level dropping too
low to cause a charging failure. It is also used for charger removal
detection and requires VIN_MIN being set to 100mV above VDD_MAX. This
causes the low charging efficiency and high power dissipation when
charging a low battery at constant charging stage.

The collapsible charger feature is introduced to improve this. It
disables VIN_MIN comparator and allows USBIN collapse to a voltage
little above to VBAT during constant charging. The CHG_GONE comparator
is used to detect the charger removal when VBUS voltage drop below VBAT
+ 80mV.

CRs-Fixed: 816844
Change-Id: I98c8f8d1d85232e898796ab0e67228334a5ac0b4
Signed-off-by: Chunmei Cai <ccai@codeaurora.org>
Signed-off-by: Fenglin Wu <fenglinw@codeaurora.org>