timer.h: add mono_time_diff_microseconds()

The current way to get a simple mono_time difference is:
1. Declare a rela_time struct
2. Assign it the value of mono_time_diff(t1, t2)
3. Get microseconds from it using rela_time_in_microseconds().

This patch adds a simpler method. Now one only needs to call
mono_time_diff_microseconds(t1, t2) to obtain the same value which
is produced from the above three steps.

Change-Id: Ibfc9cd211e48e8e60a0a7703bff09cee3250e88b
Signed-off-by: David Hendricks <dhendrix@chromium.org>
Reviewed-on: http://review.coreboot.org/3190
Tested-by: build bot (Jenkins)

src/include/timer.h

@@ -155,4 +155,12 @@ static inline long rela_time_in_microseconds(const struct rela_time *rt)
 	return rt->microseconds;
 }
 
+static inline long mono_time_diff_microseconds(const struct mono_time *t1,
+					       const struct mono_time *t2)
+{
+	struct rela_time rt;
+	rt = mono_time_diff(t1, t2);
+	return rela_time_in_microseconds(&rt);
+}
+
 #endif /* TIMER_H */