Create module dmamux.rs as common interface for DMA request mapping.

The new stm32l4+ devices use a separate peripheral (DMAMUX) for routing
a DMA request line between peripherals and DMA controllers, whereas the
"old" stm32l4 devices use the `CSELR` register for request mapping.
This module tries to abstract this difference, and provides a common
interface for all L4 devices.
This commit also enable the DMAMUX clock when required.

Author: jspngh

src/dma.rs

@@ -1082,6 +1082,18 @@ macro_rules! dma {
                     fn split(self, ahb: &mut AHB1) -> Channels {
                         <$DMAX>::enable(ahb);
 
+                        #[cfg(any(
+                            // feature = "stm32l4p5",
+                            // feature = "stm32l4q5",
+                            // feature = "stm32l4r5",
+                            // feature = "stm32l4s5",
+                            // feature = "stm32l4r7",
+                            // feature = "stm32l4s7",
+                            feature = "stm32l4r9",
+                            feature = "stm32l4s9"
+                        ))]
+                        ahb.enr().modify(|_, w| w.dmamux1en().set_bit());
+
                         // reset the DMA control registers (stops all on-going transfers)
                         $(
                             self.$ccrX.reset();