doc: dt: bindings: usb: dwc3: Update entries for disabling U1 and U2

This patch updates the documentation with the information related
to the quirks that needs to be added for disabling the link entering
into the U1 and U2 states

Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Anurag Kumar Vulisha <anurag.kumar.vulisha@xilinx.com>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>