stm32:rng: f4 rng peripheral is common with at least f2

Pull it up as common code immediately. Rename to v1, extract to common
with a doxygen marker stubs, add to F2 makefiles.

lib/stm32/f4/rng.c

@@ -1,18 +1,15 @@
+/* This file is used for documentation purposes. It does not need
+to be compiled. All source code is in the common area.
+If there is any device specific code required it can be included here,
+in which case this file must be added to the compile list. */
+
 /** @defgroup rng_file RNG
- *
- * @ingroup STM32F4xx
- *
- * @brief <b>libopencm3 STM32F4xx RNG</b>
- *
- * @version 1.0.0
- *
- * @date 25 March 2015
- *
- * This library supports the random number generator peripheral (RNG) in the
- * STM32F4 series of ARM Cortex Microcontrollers by ST Microelectronics.
- *
- *LGPL License Terms @ref lgpl_license
- */
+
+@ingroup STM32F4xx
+
+@brief <b>libopencm3 STM32F4xx RNG</b>
+
+*/
 
 /*
  * This file is part of the libopencm3 project.
@@ -32,21 +29,3 @@
  */
 
 #include <libopencm3/stm32/rng.h>
-
-/**@{*/
-
-/** Disable the Random Number Generator peripheral.
-*/
-void rng_disable(void)
-{
-    RNG_CR &= ~RNG_CR_RNGEN;
-}
-
-/** Enable the Random Number Generator peripheral.
-*/
-void rng_enable(void)
-{
-    RNG_CR |= RNG_CR_RNGEN;
-}
-
-/**@}*/