usb: dwc3: Allow disabling of metastability workaround

Some platforms (e.g. TI's DRA7 USB2 instance) have more trouble
with the metastability workaround as it supports only
a High-Speed PHY and the PHY can enter into an Erratic state [1]
when the controller is set in SuperSpeed mode as part of
the metastability workaround.

This causes upto 2 seconds delay in enumeration on DRA7's USB2
instance in gadget mode.

If these platforms can be better off without the workaround,
provide a device tree property to suggest that so the workaround
is avoided.

[1] Device mode enumeration trace showing PHY Erratic Error.
     irq/90-dwc3-969   [000] d...    52.323145: dwc3_event: event (00000901): Erratic Error [U0]
     irq/90-dwc3-969   [000] d...    52.560646: dwc3_event: event (00000901): Erratic Error [U0]
     irq/90-dwc3-969   [000] d...    52.798144: dwc3_event: event (00000901): Erratic Error [U0]

Signed-off-by: Roger Quadros <rogerq@ti.com>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>