Merge pull request #14794 from bergzand/pr/stm32/dma_add_unsuported_trigger_define

stm32: Add define for when DMA channel selection is not supported

cpu/stm32/include/periph_cpu.h

@@ -441,6 +441,12 @@ typedef enum {
     DMA_MEM_TO_MEM = 2,        /**< Memory to memory */
 } dma_mode_t;
 
+/**
+ * @brief   DMA channel/trigger configuration for DMA peripherals without
+ *          channel/trigger filtering such as the stm32f1 and stm32f3.
+ */
+#define DMA_CHAN_CONFIG_UNSUPPORTED  (UINT8_MAX)
+
 /**
  * @name    DMA Increment modes
  * @{