usb: dwc3: Add DWC_usb31 GTXTHRCFG reg fields

Add new GTXTHRCFG bit field macros for DWC_usb31. The GTXTHRCFG register
fields for DWC_usb31 is as follows:
 +-------+--------------------------+-----------------------------------+
 | BITS  | Name                     | Description                       |
 +=======+==========================+===================================+
 | 31:27 | reserved                 |                                   |
 | 26    | UsbTxPktCntSel           | Async ESS transmit packet         |
 |       |                          | threshold enable                  |
 | 25:21 | UsbTxPktCnt              | Async ESS transmit packet         |
 |       |                          | threshold count                   |
 | 20:16 | UsbMaxTxBurstSize        | Async ESS Max transmit burst size |
 | 15    | UsbTxThrNumPktSel_HS_Prd | HS high bandwidth periodic        |
 |       |                          | transmit packet threshold enable  |
 | 14:13 | UsbTxThrNumPkt_HS_Prd    | HS high bandwidth periodic        |
 |       |                          | transmit packet threshold count   |
 | 12:11 | reserved                 |                                   |
 | 10    | UsbTxThrNumPktSel_Prd    | Periodic ESS transmit packet      |
 |       |                          | threshold enable                  |
 | 9:5   | UsbTxThrNumPkt_Prd       | Periodic ESS transmit packet      |
 |       |                          | threshold count                   |
 | 4:0   | UsbMaxTxBurstSize_Prd    | Max periodic ESS TX burst size    |
 +-------+--------------------------+-----------------------------------+

Signed-off-by: Thinh Nguyen <thinhn@synopsys.com>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>