imx: boot_mode: add USB_BOOT entry

Add USB_BOOT entry.

Signed-off-by: Peng Fan <peng.fan@nxp.com>
Reviewed-by: Stefano Babic <sbabic@denx.de>
Reviewed-by: Fabio Estevam <fabio.estevam@nxp.com>
diff --git a/arch/arm/include/asm/mach-imx/boot_mode.h b/arch/arm/include/asm/mach-imx/boot_mode.h
index a8239f2..e3ed046 100644
--- a/arch/arm/include/asm/mach-imx/boot_mode.h
+++ b/arch/arm/include/asm/mach-imx/boot_mode.h
@@ -26,6 +26,7 @@
 	MMC4_BOOT,
 	NAND_BOOT,
 	QSPI_BOOT,
+	USB_BOOT,
 	UNKNOWN_BOOT,
 	BOOT_DEV_NUM = UNKNOWN_BOOT,
 };