stm32: flash: pull up prefetch to _all

Turns out, there's lots of common code for flash.  Pull up prefetch
on/off to start with, as there's only a single bit name different.

Pull up the definitions of common API functions too, starting with
flash_set_ws.  Even if the implementations are different, things that
meant to be the same, should be defined centrally.

lib/stm32/l4/flash.c

@@ -42,31 +42,6 @@
 
 #include <libopencm3/stm32/flash.h>
 
-/** @brief Enable the FLASH Prefetch Buffer
-
-This buffer is used for instruction fetches and is enabled by default after
-reset.
-
-Note carefully the clock restrictions under which the prefetch buffer may be
-enabled or disabled. Changes are normally made while the clock is running in
-the power-on low frequency mode before being set to a higher speed mode.
-See the reference manual for details.
-*/
-void flash_prefetch_enable(void)
-{
-	FLASH_ACR |= FLASH_ACR_PRFTEN;
-}
-
-/** @brief Disable the FLASH Prefetch Buffer
-
-Note carefully the clock restrictions under which the prefetch buffer may be
-set to disabled. See the reference manual for details.
-*/
-void flash_prefetch_disable(void)
-{
-	FLASH_ACR &= ~FLASH_ACR_PRFTEN;
-}
-
 /** @brief Set the Number of Wait States
 
 Used to match the system clock to the FLASH memory access time. See the