board: arduino-mega2560: add xtimer configuration values

boards/arduino-mega2560/include/board.h

@@ -73,6 +73,14 @@ extern "C" {
 #define LED_RED_TOGGLE      /* not available */
 /** @} */
 
+/**
+ * @brief xtimer configuration values
+ * @{
+ */
+#define XTIMER_SHIFT                (2)
+#define XTIMER_SHIFT_ON_COMPARE     (8)
+#define XTIMER_BACKOFF              (40)
+/** @} */
 
 /**
  * @brief Initialize board specific hardware, including clock, LEDs and std-IO