lintian: Add a long-form description to satisfy lintian

Change-Id: Id912df43c25b06ddcb36aff8e512d010e00d6796
diff --git a/debian/control b/debian/control
index b9e0910..c3f37f2 100644
--- a/debian/control
+++ b/debian/control
@@ -9,3 +9,4 @@
 Priority: optional
 Architecture: arm64 armhf
 Description: Arm Trusted Framework for IMX platform
+ This contains the Arm Trusted Framework for the NXP i.MX platform.