power: pm8921-charger: new procedure for reverse boost fix

The current solution disables the voltage regulation if the usb
source is 500mA. This essentially removes support for a source
that provides less than 500mA.

Instead use this workaround
1. Increase the minimum pfet on time to 100uS.
2. Increase vin_min by 200mV
3. Wait 250mS
4. restore vin_min, and the minimum pfet on time to 25uS.

Change-Id: I333c810568cdc656d2e13f0eeb9511857335b834
Signed-off-by: Abhijeet Dharmapurikar <adharmap@codeaurora.org>