tests: disk: disk_access: test STM32L4 DMA

Ensure that the shared DMA variant of the STM SDMMC driver is compiled
in CI.

Signed-off-by: Jordan Yates <[email protected]>

Author: JordanYates

tests/drivers/disk/disk_access/boards/stm32l496g_disco_stm32l496xx.conf

@@ -0,0 +1 @@
+CONFIG_DMA=y

tests/drivers/disk/disk_access/boards/stm32l496g_disco_stm32l496xx.overlay

@@ -0,0 +1,7 @@
+/* SPDX-License-Identifier: Apache-2.0 */
+
+&sdmmc1 {
+	/* Channel 5 request 7 on DMA2 */
+	dmas = <&dma2 5 7 STM32_DMA_PRIORITY_HIGH>;
+	dma-names = "txrx";
+};