arm/dts: 8974: Add qpnp-vadc phandle for usb3 device node

Add qpnp-vadc phandle for usb3 device node which is required
to get usb power supply's VOLTAGE_NOW property.

CRs-Fixed: 599547
Change-Id: Ia01e0620ec9a6ca28adb9e71ccac030a33a41642
Signed-off-by: Vijayavardhan Vennapusa <vvreddy@codeaurora.org>