usb: dwc3-msm: Get usb power_supply from device tree

Not all instances of the controller should be tied to the
USB power_supply. For example in cases of multiple instances,
we do not want all instances manipulating the SDP current
draw by setting the power_supply property, as there is only
usually only a single charger instance.

Replace power_supply lookup by "usb" name to a phandle lookup.
DT must specify a "qcom,usb-charger" property pointing to the
charger instance.

Change-Id: Ibd90337782b23f5e1a1a1c2af5a48c49be37823d
Signed-off-by: Jack Pham <jackp@codeaurora.org>