stm32: doc: pwr: move to peripheral_api

Some families had partially moved to peripheral api, and others were
only documenting common code, but not specific code.  Delete dummy .c
files, and check that all specific apis are also being documented, not
just common apis.

lib/stm32/common/pwr_common_v1.c

@@ -1,4 +1,5 @@
-/** @addtogroup pwr_file
+/** @addtogroup pwr_file PWR peripheral API
+@ingroup peripheral_apis
 
 @author @htmlonly © @endhtmlonly 2012
 Ken Sarkies <ksarkies@internode.on.net>