pk: make mbedtls_pk_ec internal when !ECP_C

mbedtls_pk_ec() is not an ideal function because:
- it provides direct access to the ecp_keypair structure wrapped
  by the pk_context and
- this bypasses the PK module's control
However, since for backward compatibility, it cannot be deprecated
immediately, 2 alternative internal functions are proposed.
As a consequence:
- when ECP_C is defined, then the legacy mbedtls_pk_ec is available
- when only ECP_LIGHT is defined, but ECP_C is not, then only the
  new internal functions will be available

Signed-off-by: Valerio Setti <valerio.setti@nordicsemi.no>

include/mbedtls/pk.h

@@ -778,7 +778,7 @@ static inline mbedtls_rsa_context *mbedtls_pk_rsa(const mbedtls_pk_context pk)
 }
 #endif /* MBEDTLS_RSA_C */
 
-#if defined(MBEDTLS_ECP_LIGHT)
+#if defined(MBEDTLS_ECP_C)
 /**
  * Quick access to an EC context inside a PK context.
  *
@@ -801,7 +801,7 @@ static inline mbedtls_ecp_keypair *mbedtls_pk_ec(const mbedtls_pk_context pk)
             return NULL;
     }
 }
-#endif /* MBEDTLS_ECP_LIGHT */
+#endif /* MBEDTLS_ECP_C */
 
 #if defined(MBEDTLS_PK_PARSE_C)
 /** \ingroup pk_module */