Rework incorrect use of assert() and panic() in codebase

Assert a valid security state using the macro sec_state_is_valid().
Replace assert() with panic() in those cases that might arise
because of runtime errors and not programming errors.
Replace panic() with assert() in those cases that might arise
because of programming errors.

Fixes ARM-software/tf-issues#96

Change-Id: I51e9ef0439fd5ff5e0edfef49050b69804bf14d5
diff --git a/include/drivers/arm/tzc400.h b/include/drivers/arm/tzc400.h
index b4aa3ba..03fce54 100644
--- a/include/drivers/arm/tzc400.h
+++ b/include/drivers/arm/tzc400.h
@@ -90,6 +90,7 @@
 #define GATE_KEEPER_OS_MASK	0xf
 #define GATE_KEEPER_OR_SHIFT	0
 #define GATE_KEEPER_OR_MASK	0xf
+#define GATE_KEEPER_FILTER_MASK	0x1
 
 /* Speculation is enabled by default. */
 #define SPECULATION_CTRL_WRITE_DISABLE	(1 << 1)