imx: boot_mode: add USB_BOOT entry

Add USB_BOOT entry.
Signed-off-by: NPeng Fan <peng.fan@nxp.com>
Reviewed-by: NStefano Babic <sbabic@denx.de>
Reviewed-by: NFabio Estevam <fabio.estevam@nxp.com>

arch/arm/include/asm/mach-imx/boot_mode.h

@@ -26,6 +26,7 @@ enum boot_device {
 	MMC4_BOOT,
 	NAND_BOOT,
 	QSPI_BOOT,
+	USB_BOOT,
 	UNKNOWN_BOOT,
 	BOOT_DEV_NUM = UNKNOWN_BOOT,
 };