armv8: fsl-layerscape: Fix SECURE_BOOT config

Without a prompt in Kconfig, SECURE_BOOT cannot be selected by
defconfig. The option was dropped unintentionally when defconfig
files were cleaned up. Three targets were impacted
ls1043ardb_SECURE_BOOT, ls2080ardb_SECURE_BOOT,
ls2080aqds_SECURE_BOOT.

Signed-off-by: York Sun <york.sun@nxp.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
diff --git a/arch/arm/cpu/armv8/fsl-layerscape/Kconfig b/arch/arm/cpu/armv8/fsl-layerscape/Kconfig
index 92e8fa6..79e2dd6 100644
--- a/arch/arm/cpu/armv8/fsl-layerscape/Kconfig
+++ b/arch/arm/cpu/armv8/fsl-layerscape/Kconfig
@@ -131,7 +131,7 @@
 	  in spin table to properly handle all cores.
 
 config SECURE_BOOT
-	bool
+	bool "Secure Boot"
 	help
 		Enable Freescale Secure Boot feature