spl: define BOOT_DEVICE_USB

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

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>

arch/arm/include/asm/spl.h

@@ -24,6 +24,7 @@ enum {
 	BOOT_DEVICE_NOR,
 	BOOT_DEVICE_UART,
 	BOOT_DEVICE_SPI,
+	BOOT_DEVICE_USB,
 	BOOT_DEVICE_SATA,
 	BOOT_DEVICE_I2C,
 	BOOT_DEVICE_BOARD,