usb: dwc3: Update VBUS status with USB controller

It is required to update VBUS status to USB controller using qscratch
registers HS_PHY_CTRL and SS_PHY_CTRL interfacing high-speed and
super-speed PHYs. This change perfoms same from USB controller's glue
driver on starting and stopping peripheral mode based on supported
USB speed with USB gadget.

It also updates devicetree documentation explicitly mentioning required
register sets.

CRs-Fixed: 1046503
Change-Id: I92df87c0e2ff54dd7ee513d277cc075eab561019
Signed-off-by: Mayank Rana <mrana@codeaurora.org>