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/f4/pwr.c

@@ -1,6 +1,6 @@
-/** @defgroup pwr_file PWR
+/** @defgroup pwr_file
  *
- * @ingroup STM32F4xx
+ * @ingroup peripheral_apis
  *
  * @brief <b>libopencm3 STM32F4xx Power Control</b>
  *
@@ -36,6 +36,8 @@
 
 #include <libopencm3/stm32/pwr.h>
 
+/**@{*/
+
 void pwr_set_vos_scale(enum pwr_vos_scale scale)
 {
 	uint32_t reg32;
@@ -43,3 +45,5 @@ void pwr_set_vos_scale(enum pwr_vos_scale scale)
 	reg32 |= (scale & PWR_CR_VOS_MASK) << PWR_CR_VOS_SHIFT;
 	PWR_CR = reg32;
 }
+
+/**@}*/