feat(auth): ecdsa p384 key support

Use KEY_SIZE 384 to enable ECDSA P384 key support by
setting MBEDTLS_ECP_DP_SECP384R1_ENABLED.

Selected by setting KEY_ALG=ecdsa and KEY_SIZE=384.

Change-Id: I382f34fc4da98f166a2aada5d16fdf44632b47f5
Signed-off-by: Lauren Wehrmeister <lauren.wehrmeister@arm.com>
diff --git a/drivers/auth/mbedtls/mbedtls_common.mk b/drivers/auth/mbedtls/mbedtls_common.mk
index 376b6b7..e380c86 100644
--- a/drivers/auth/mbedtls/mbedtls_common.mk
+++ b/drivers/auth/mbedtls/mbedtls_common.mk
@@ -107,11 +107,21 @@
 
 ifeq (${TF_MBEDTLS_KEY_SIZE},)
     ifneq ($(findstring rsa,${TF_MBEDTLS_KEY_ALG}),)
-	ifeq (${KEY_SIZE},)
+        ifeq (${KEY_SIZE},)
             TF_MBEDTLS_KEY_SIZE		:=	2048
-	else
+        else ifneq ($(filter $(KEY_SIZE), 1024 2048 3072 4096),)
+            TF_MBEDTLS_KEY_SIZE		:=	${KEY_SIZE}
+        else
+            $(error "Invalid value for KEY_SIZE: ${KEY_SIZE}")
+        endif
+    else ifneq ($(findstring ecdsa,${TF_MBEDTLS_KEY_ALG}),)
+        ifeq (${KEY_SIZE},)
+            TF_MBEDTLS_KEY_SIZE		:=	256
+        else ifneq ($(filter $(KEY_SIZE), 256 384),)
             TF_MBEDTLS_KEY_SIZE		:=	${KEY_SIZE}
-	endif
+        else
+            $(error "Invalid value for KEY_SIZE: ${KEY_SIZE}")
+        endif
     endif
 endif