power: qpnp-charger: add support for batfet control regulator

The battery FET (batfet) is put in low power mode (LPM) where it can
allow less than 100mA of current when the PMIC goes to sleep.

This causes problems if high current discharge from the battery
is expected when the system (and the PMIC) go to sleep. Override
this LPM feature and let the batfet remain in high power mode (HPM) if a
high current discharge is expected.

Expose the batfet as a regulator and let the consumers decide whether
to enable or disable this regulator. Enabled means the batfet will stay
in HPM while PMIC goes to sleep whereas disabled means the batfet will
be switched to LPM when the PMIC goes to sleep.

CRs-Fixed: 503417
Change-Id: I883271f17ed04b37016afc9c3ea89235bde7af88
Signed-off-by: Abhijeet Dharmapurikar <adharmap@codeaurora.org>