boards/mulle: Reduce RTC crystal load capacitance

boards/mulle/include/periph_conf.h

@@ -793,8 +793,12 @@ extern "C"
 /**
  * RTC module crystal load capacitance configuration bits.
  */
-/* enable 12pF load capacitance, might need adjusting.. */
-#define RTT_LOAD_CAP_BITS   (RTC_CR_SC8P_MASK | RTC_CR_SC4P_MASK)
+/* The crystal on the Mulle is designed for 12.5 pF load capacitance. According
+ * to the data sheet, the K60 will have a 5 pF parasitic capacitance on the
+ * XTAL32/EXTAL32 connection. The board traces might give some minor parasitic
+ * capacitance as well. */
+/* enable 6pF load capacitance, might need adjusting.. */
+#define RTT_LOAD_CAP_BITS   (RTC_CR_SC4P_MASK | RTC_CR_SC2P_MASK | RTC_CR_SC1P_MASK)
 /** @} */