Added functions and example for deep sleep on mb851.

The added function let a process to make the system go
into deep sleep for maximum power saving.
The udp-ipv6-example shows how to use these new functions.

cpu/stm32w108/sleep.h

@@ -0,0 +1,10 @@
+
+/* Enter system in deep sleep 1 (core power domain is fully
+ * powered down and sleep timer is active).
+ * Execution is suspended for a given number of seconds.
+ *
+ * Pay attention! All system peripherals (including sensors) have
+ * to be reinitialized before being used again. UART, LEDs and
+ * real timers are automatically reinitialized. */
+
+void sleep_seconds(int seconds);