usb: dwc3: add tracepoints to aid debugging

When we're debugging hard-to-reproduce and time-sensitive
use cases, printk() poses too much overhead. That's when
the kernel's tracing infrastructure comes into play.

This patch implements a few initial tracepoints for the
dwc3 driver. More traces can be added as necessary in order
to ease the task of debugging dwc3.

Reviewed-by: Paul Zimmerman <paulz@synopsys.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>
diff --git a/drivers/usb/dwc3/debug.h b/drivers/usb/dwc3/debug.h
index 12ff4c9..07fbc2d 100644
--- a/drivers/usb/dwc3/debug.h
+++ b/drivers/usb/dwc3/debug.h
@@ -214,6 +214,8 @@
 	}
 }
 
+void dwc3_trace(void (*trace)(struct va_format *), const char *fmt, ...);
+
 #ifdef CONFIG_DEBUG_FS
 extern int dwc3_debugfs_init(struct dwc3 *);
 extern void dwc3_debugfs_exit(struct dwc3 *);