timer: Tidy up use of notrace

Tracing is typically enabled by the time driver model starts up, so there
is no point in adding a 'notrace' to the timer-init function. However,
once the driver model timer is enabled, we do need to be able to access
the timer's private data when reading the timer, so add it to the core
function needed for that.

Update the function's documentation while we are here.

Signed-off-by: Simon Glass <sjg@chromium.org>

include/timer.h

@@ -9,11 +9,16 @@
 #define timer_get_ops(dev)	((struct timer_ops *)(dev)->driver->ops)
 
 /**
- * dm_timer_init() - initialize a timer for time keeping. On success
- * initializes gd->timer so that lib/timer can use it for future
- * referrence.
+ * dm_timer_init() - set up a timer for time keeping
  *
- * Return: 0 on success or error number
+ * Sets up gd->timer if the device is not already bound, making sure it is
+ * probed and ready for use
+ *
+ * On success, inits gd->timer so that lib/timer can use it for future reference
+ *
+ * Returns: 0 on success, -EAGAIN if driver model is not ready yet, -ENODEV if
+ * no timer could be found, other error if the timer could not be bound or
+ * probed
  */
 int dm_timer_init(void);