ColdFire: Fix compilation issue caused by a missing function

Implement usec2ticks() which is used by fsl_i2c.c in
lib_m68k/time.c

Signed-off-by: TsiChung Liew <Tsi-Chung.Liew@freescale.com>

lib_m68k/time.c

@@ -199,6 +199,11 @@ unsigned long long get_ticks(void)
 	return get_timer(0);
 }
 
+unsigned long usec2ticks(unsigned long usec)
+{
+	return get_timer(usec);
+}
+
 /*
  * This function is derived from PowerPC code (timebase clock frequency).
  * On M68K it returns the number of timer ticks per second.