imx: hab: Explain that ivt_offset is optional

The ivt_offset parameter is optional for both hab_auth_img_or_fail
and hab_auth_img commands.

Document it in their usage texts to make it clearer.

Signed-off-by: Fabio Estevam <festevam@denx.de>
diff --git a/arch/arm/mach-imx/hab.c b/arch/arm/mach-imx/hab.c
index e7b0388..27e053e 100644
--- a/arch/arm/mach-imx/hab.c
+++ b/arch/arm/mach-imx/hab.c
@@ -703,7 +703,7 @@
 		"addr length ivt_offset\n"
 		"addr - image hex address\n"
 		"length - image hex length\n"
-		"ivt_offset - hex offset of IVT in the image"
+		"ivt_offset - hex offset of IVT in the image (optional)"
 	  );
 
 U_BOOT_CMD(
@@ -719,7 +719,7 @@
 		"addr length ivt_offset\n"
 		"addr - image hex address\n"
 		"length - image hex length\n"
-		"ivt_offset - hex offset of IVT in the image"
+		"ivt_offset - hex offset of IVT in the image (optional)"
 	  );
 
 U_BOOT_CMD(