usb: dwc3: Make TX/RX threshold configurable

DWC_usb31 periodic transfer at 48K+ bytes per interval may need
modification to the TX/RX packet threshold to achieve optimal result.
Add properties to make it configurable.

By default, periodic ESS TX and RX threshold are not enabled. To enable
TX or RX threshold (host mode only), both packet threshold count and max
burst size properties must be set to a valid non-zero value 1-16.

DWC_usb31 programming guide section 1.2.3 and 1.2.4.

Cc: John Youn <johnyoun@synopsys.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Thinh Nguyen <thinhn@synopsys.com>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>