STM32 DMA: replaced F4 header to match that for F2, and move header and source files to common.

In the header:
- Fix DMA_SxCR_CT: change shift from 18 to 19
For use with the convenience functions:
- Added DMA_SxCR_CHSEL generic values
- Added DMA_STREAM - generic values
- Added dma_if_offset - rather than separating out LISR and HISR
- Added masks
- Added dma_disable_double_buffer_mode()

include/libopencm3/stm32/dma.h

@@ -0,0 +1,31 @@
+/* This provides unification of code over STM32F subfamilies */
+
+/*
+ * 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/>.
+ */
+
+#if defined(STM32F1)
+#       include <libopencm3/stm32/f1/dma.h>
+#elif defined(STM32F2)
+#       include <libopencm3/stm32/f2/dma.h>
+#elif defined(STM32F4)
+#       include <libopencm3/stm32/f4/dma.h>
+/*#elif defined(STM32L1)
+#       include <libopencm3/stm32/l1/dma.h>*/
+#else
+#       error "stm32 family not defined."
+#endif
+