dt-bindings: document a new quirk for dwc3

At Hikey 970, setting the SPLIT disable at the General
User Register 3 is required.

Without that, the URBs generated by the usbhid driver
return -EPROTO errors. That causes the code at
hid-core.c to call hid_io_error(), which schedules
a reset_work, causing a call to hid_reset().

In turn, the code there will call:

	usb_queue_reset_device(usbhid->intf);

The net result is that the input devices won't work, and
will be reset on every 0.5 seconds:

	[   33.122384] hub 1-1:1.0: state 7 ports 4 chg 0000 evt 0002
	[   33.378220] usb 1-1.1: reset low-speed USB device number 3 using xhci-hcd
	[   33.698394] hub 1-1:1.0: state 7 ports 4 chg 0000 evt 0000
	[   34.882365] hub 1-1:1.0: state 7 ports 4 chg 0000 evt 0002
	[   35.138217] usb 1-1.1: reset low-speed USB device number 3 using xhci-hcd
	[   35.458617] hub 1-1:1.0: state 7 ports 4 chg 0000 evt 0000
	[   36.642392] hub 1-1:1.0: state 7 ports 4 chg 0000 evt 0002
	[   36.898207] usb 1-1.1: reset low-speed USB device number 3 using xhci-hcd
	[   37.218598] hub 1-1:1.0: state 7 ports 4 chg 0000 evt 0000
	[   38.402368] hub 1-1:1.0: state 7 ports 4 chg 0000 evt 0002
	[   38.658174] usb 1-1.1: reset low-speed USB device number 3 using xhci-hcd
	[   38.978594] hub 1-1:1.0: state 7 ports 4 chg 0000 evt 0000
	[   40.162361] hub 1-1:1.0: state 7 ports 4 chg 0000 evt 0002
	[   40.418148] usb 1-1.1: reset low-speed USB device number 3 using xhci-hcd
	...
	[  397.698132] usb 1-1.1: reset low-speed USB device number 3 using xhci-hcd
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Signed-off-by: Felipe Balbi <balbi@kernel.org>

Documentation/devicetree/bindings/usb/dwc3.txt

@@ -78,6 +78,9 @@ Optional properties:
 			park mode are disabled.
  - snps,dis_metastability_quirk: when set, disable metastability workaround.
 			CAUTION: use only if you are absolutely sure of it.
+ - snps,dis-split-quirk: when set, change the way URBs are handled by the
+			 driver. Needed to avoid -EPROTO errors with usbhid
+			 on some devices (Hikey 970).
  - snps,is-utmi-l1-suspend: true when DWC3 asserts output signal
 			utmi_l1_suspend_n, false when asserts utmi_sleep_n
  - snps,hird-threshold: HIRD threshold