Add trace library

Add a library which supports tracing of execution using built-in gcc
features and a microsecond timer. This can be used to record a list of
function which are executed, along with a timestamp for each. Later
this information can be sent to the host for processing.

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

include/common.h

@@ -750,6 +750,10 @@ void	irq_install_handler(int, interrupt_handler_t *, void *);
 void	irq_free_handler   (int);
 void	reset_timer	   (void);
 ulong	get_timer	   (ulong base);
+
+/* Return value of monotonic microsecond timer */
+unsigned long timer_get_us(void);
+
 void	enable_interrupts  (void);
 int	disable_interrupts (void);