power: pm8921-charger: Add pm8921_set_usb_power_supply_type

USB can distinguish between types of USB connections such as a
DC charger or PC connection.

Let the USB drivers set the specifc power_supply_type via this
function.

CRs-Fixed: 326548
Change-Id: I31954b3f8d2b1e5fd72712e1f94f67fdbc2e205d
Signed-off-by: David Keitel <dkeitel@codeaurora.org>