imx8mm_evk: Add an entry for USB boot

Add an entry for USB boot so that U-Boot could be loaded via
the Serial Download Protocol.

Signed-off-by: Fabio Estevam <festevam@denx.de>
Reviewed-by: Peng Fan <peng.fan@nxp.com>
diff --git a/board/freescale/imx8mm_evk/spl.c b/board/freescale/imx8mm_evk/spl.c
index e0e97c7..b5a2faf 100644
--- a/board/freescale/imx8mm_evk/spl.c
+++ b/board/freescale/imx8mm_evk/spl.c
@@ -33,6 +33,8 @@
 int spl_board_boot_device(enum boot_device boot_dev_spl)
 {
 	switch (boot_dev_spl) {
+	case USB_BOOT:
+		return BOOT_DEVICE_BOARD;
 	case SD2_BOOT:
 	case MMC2_BOOT:
 		return BOOT_DEVICE_MMC1;