drivers: crypto: Add authenticated decryption framework

Add framework for autheticated decryption of data. Currently this
patch optionally imports mbedtls library as a backend if build option
"DECRYPTION_SUPPORT = aes_gcm" is set to perform authenticated decryption
using AES-GCM algorithm.

Signed-off-by: Sumit Garg <sumit.garg@linaro.org>
Change-Id: I2966f0e79033151012bf4ffc66f484cd949e7271
diff --git a/make_helpers/defaults.mk b/make_helpers/defaults.mk
index 9273469..0127607 100644
--- a/make_helpers/defaults.mk
+++ b/make_helpers/defaults.mk
@@ -65,6 +65,9 @@
 # Debug build
 DEBUG				:= 0
 
+# By default disable authenticated decryption support.
+DECRYPTION_SUPPORT		:= none
+
 # Build platform
 DEFAULT_PLAT			:= fvp