usb: Add LED triggers for USB activity

With this patch, USB activity can be signaled by blinking a LED. There
are two triggers, one for activity on USB host and one for USB gadget.

Both triggers should work with all host/device controllers. Tested only
with musb.

Performace: I measured performance overheads on ARM Cortex-A8 (TI
AM335x) running on 600 MHz.

Duration of usb_led_activity():
- with no LED attached to the trigger:        2 ± 1 µs
- with one GPIO LED attached to the trigger:  2 ± 1 µs or 8 ± 2 µs (two peaks in histogram)

Duration of functions calling usb_led_activity() (with this patch
applied and no LED attached to the trigger):
- __usb_hcd_giveback_urb():    10 - 25 µs
- usb_gadget_giveback_request(): 2 - 6 µs

Signed-off-by: Michal Sojka <sojka@merica.cz>
Acked-by: Felipe Balbi <balbi@ti.com>
Tested-by: Felipe Balbi <balbi@ti.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

include/linux/usb.h

@@ -1862,6 +1862,18 @@ extern void usb_unregister_notify(struct notifier_block *nb);
 /* debugfs stuff */
 extern struct dentry *usb_debug_root;
 
+/* LED triggers */
+enum usb_led_event {
+	USB_LED_EVENT_HOST = 0,
+	USB_LED_EVENT_GADGET = 1,
+};
+
+#ifdef CONFIG_USB_LED_TRIG
+extern void usb_led_activity(enum usb_led_event ev);
+#else
+static inline void usb_led_activity(enum usb_led_event ev) {}
+#endif
+
 #endif  /* __KERNEL__ */
 
 #endif