usb: dwc3: rockchip: support to set testmodes via debugfs

usb: dwc3: rockchip: support to set testmodes via debugfs

This patch create host_testmode file in debugfs for
USB HOST. It's useful for us to use a scope to verify
signal integrity for USB2/USB3 HOST.

For example， set testmodes for RK3399 board USB:
1. set Test packet for Type-C0 USB2 HOST:
   echo test_packet > /sys/kernel/debug/usb@fe800000/host_testmode

2. set compliance mode for Type-C0 USB3 HOST normal orientation:
   echo test_u3 > /sys/kernel/debug/usb@fe800000/host_testmode

3. set compliance mode for Type-C0 USB3 HOST flip orientation:
   echo test_flip_u3 > /sys/kernel/debug/usb@fe800000/host_testmode

4. check the testmode status:
   cat /sys/kernel/debug/usb@fe800000/host_testmode

   The log maybe like this:
   U2: test_packet     /* means that U2 in test mode */
   U3: compliance mode /* means that U3 in test mode */

Change-Id: Ic7e464b0443c792848846246b782ffba30bf2120
Signed-off-by: William Wu <wulf@rock-chips.com>