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/common/flash_common_all.c

@@ -0,0 +1,37 @@
+/** @addtogroup flash_file
+ *
+ */
+
+/*
+ * This file is part of the libopencm3 project.
+ *
+ * This library is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU Lesser General Public License as published by
+ * the Free Software Foundation, either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * This library is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU Lesser General Public License for more details.
+ *
+ * You should have received a copy of the GNU Lesser General Public License
+ * along with this library.  If not, see <http://www.gnu.org/licenses/>.
+ */
+
+/**@{*/
+
+#include <libopencm3/stm32/flash.h>
+
+void flash_prefetch_enable(void)
+{
+	FLASH_ACR |= FLASH_ACR_PRFTEN;
+}
+
+void flash_prefetch_disable(void)
+{
+	FLASH_ACR &= ~FLASH_ACR_PRFTEN;
+}
+
+
+/*@}*/