power: pm8921-charger: set vddmax in increments of step size

When charging a dead battery and increasing VDD_MAX
voltage there may be an inrush current causing USB_IN to
severely drop causing the device to reset.

To avoid this severe drop add a method to increase
or decrease VDD_MAX only in increments of 20 mV until
desired voltage has been reached.

Change-Id: Idf522ac837c75bce7b5a2b3e870696afe29eabf3
Signed-off-by: David Keitel <dkeitel@codeaurora.org>