libmbedtls: support mbedtls DH function

Implement DH function based on mbedtls.

Acked-by: Etienne Carriere <etienne.carriere@linaro.org>
Signed-off-by: Edison Ai <edison.ai@arm.com>
Signed-off-by: Jens Wiklander <jens.wiklander@linaro.org>
diff --git a/lib/libmbedtls/include/mbedtls_config_kernel.h b/lib/libmbedtls/include/mbedtls_config_kernel.h
index b1950db..b3d71cd 100644
--- a/lib/libmbedtls/include/mbedtls_config_kernel.h
+++ b/lib/libmbedtls/include/mbedtls_config_kernel.h
@@ -84,6 +84,10 @@
 #define MBEDTLS_ASN1_WRITE_C
 #endif
 
+#if defined(CFG_CRYPTO_DH)
+#define MBEDTLS_DHM_C
+#endif
+
 #endif /*CFG_CRYPTOLIB_NAME_mbedtls*/
 
 #include <mbedtls/check_config.h>