[stm32] Support the "new" BCD style RTC peripheral

Add the register definitions and some of the most basic helper functions
for the new style BCD RTC module found on the F2, F4, L1, F3 and F0.

This tries to keep as close to HACKING_COMMON_DOC as possible, while
maintaining sane names.

include/libopencm3/stm32/f1/rtc.h

@@ -17,6 +17,11 @@
  * along with this library.  If not, see <http://www.gnu.org/licenses/>.
  */
 
+/*
+ * The F1 RTC is a straight timestamp, a completely different peripheral to
+ * that found in the F2, F3, F4, L1 and F0.
+ */
+
 #ifndef LIBOPENCM3_RTC_H
 #define LIBOPENCM3_RTC_H