ARM: imx: Use USB SDPS as fallback option on Data Modul i.MX8M Plus eDM SBC

The Data Modul i.MX8M Plus eDM SBC does have USB gadget capable port
accessible via USB A-A cable plugged into the bottom USB 3.0 port.
Use USB SDPS as the fallback boot device, so USB SDPS loading can
be performed using e.g. uuu tool.

Signed-off-by: Marek Vasut <marex@denx.de>
diff --git a/board/data_modul/imx8mp_edm_sbc/spl.c b/board/data_modul/imx8mp_edm_sbc/spl.c
index c193589..f81b727 100644
--- a/board/data_modul/imx8mp_edm_sbc/spl.c
+++ b/board/data_modul/imx8mp_edm_sbc/spl.c
@@ -100,7 +100,10 @@
 	if (boot_dev_spl == MMC3_BOOT)		/* eMMC */
 		return BOOT_DEVICE_MMC2;
 
-	return BOOT_DEVICE_MMC1;		/* SD */
+	if (boot_dev_spl == SD2_BOOT)		/* SD */
+		return BOOT_DEVICE_MMC1;
+
+	return BOOT_DEVICE_BOOTROM;		/* USB SDPS */
 }
 
 void board_boot_order(u32 *spl_boot_list)