spl: define BOOT_DEVICE_USB

This enum is referenced from common/spl/spl.c.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
diff --git a/arch/arm/include/asm/spl.h b/arch/arm/include/asm/spl.h
index 5c5d33f..19c38f4 100644
--- a/arch/arm/include/asm/spl.h
+++ b/arch/arm/include/asm/spl.h
@@ -24,6 +24,7 @@
 	BOOT_DEVICE_NOR,
 	BOOT_DEVICE_UART,
 	BOOT_DEVICE_SPI,
+	BOOT_DEVICE_USB,
 	BOOT_DEVICE_SATA,
 	BOOT_DEVICE_I2C,
 	BOOT_DEVICE_BOARD,