ARM: Add new BOOT_DEVICE_DFU boot mode

This enum is needed when SPL_DFU is enabled.

Signed-off-by: Michal Simek <michal.simek@xilinx.com>
diff --git a/arch/arm/include/asm/spl.h b/arch/arm/include/asm/spl.h
index 19c38f4..6f312d6 100644
--- a/arch/arm/include/asm/spl.h
+++ b/arch/arm/include/asm/spl.h
@@ -28,6 +28,7 @@
 	BOOT_DEVICE_SATA,
 	BOOT_DEVICE_I2C,
 	BOOT_DEVICE_BOARD,
+	BOOT_DEVICE_DFU,
 	BOOT_DEVICE_NONE
 };
 #endif