arm: imx: Add function to validate i.MX8 containers

Add a function to abstract the common task of validating i.MX8 container
image headers.

Signed-off-by: Sean Anderson <seanga2@gmail.com>
diff --git a/arch/arm/mach-imx/imx8/ahab.c b/arch/arm/mach-imx/imx8/ahab.c
index b58b14c..44ea635 100644
--- a/arch/arm/mach-imx/imx8/ahab.c
+++ b/arch/arm/mach-imx/imx8/ahab.c
@@ -146,7 +146,7 @@
 	}
 
 	phdr = (struct container_hdr *)addr;
-	if (phdr->tag != 0x87 && phdr->version != 0x0) {
+	if (!valid_container_hdr(phdr)) {
 		printf("Error: Wrong container header\n");
 		return -EFAULT;
 	}