Add mbedtls_ecdh_can_do

All curves can currently do ECDH, but to make the API symmetric and
future-proof, add mbedtls_ecdh_can_do() to go with mbedtls_ecdsa_can_do().

include/mbedtls/ecdh.h

@@ -144,6 +144,15 @@ typedef struct mbedtls_ecdh_context
 }
 mbedtls_ecdh_context;
 
+/**
+ * \brief          Check whether a given group can be used for ECDH.
+ *
+ * \param gid      The ECP group ID to check.
+ *
+ * \return         \c 1 if the group can be used, \c 0 otherwise
+ */
+int mbedtls_ecdh_can_do( mbedtls_ecp_group_id gid );
+
 /**
  * \brief           This function generates an ECDH keypair on an elliptic
  *                  curve.