imx: boot_mode: Add FLEXSPI boot entry

i.MX8 support FLEXSPI boot support. So add FLEXSPI boot entry.

Signed-off-by: Peng Fan <peng.fan@nxp.com>
Reviewed-by: Anatolij Gustschin <agust@denx.de>
Cc: Stefano Babic <sbabic@denx.de>
diff --git a/arch/arm/include/asm/mach-imx/boot_mode.h b/arch/arm/include/asm/mach-imx/boot_mode.h
index 8766e9d..3a483b6 100644
--- a/arch/arm/include/asm/mach-imx/boot_mode.h
+++ b/arch/arm/include/asm/mach-imx/boot_mode.h
@@ -25,6 +25,7 @@
 	MMC4_BOOT,
 	NAND_BOOT,
 	QSPI_BOOT,
+	FLEXSPI_BOOT,
 	USB_BOOT,
 	UNKNOWN_BOOT,
 	BOOT_DEV_NUM = UNKNOWN_BOOT,