ecp_curves: Update pre-processor define guards for `ecp_mpi_load()`.

This patch adjusts the logic, so that the method is included,
when the following components are enabled:

* MBEDTLS_ECP_DP_CURVE448_ENABLED
* MBEDTLS_ECP_DP_CURVE25519_ENABLED
* ECP_LOAD_GROUP

Signed-off-by: Minos Galanakis <minos.galanakis@arm.com>
diff --git a/library/ecp_curves.c b/library/ecp_curves.c
index 88c4c85..727283f 100644
--- a/library/ecp_curves.c
+++ b/library/ecp_curves.c
@@ -4502,7 +4502,9 @@
 #endif
 #endif /* MBEDTLS_ECP_DP_BP512R1_ENABLED */
 
-#if defined(ECP_LOAD_GROUP)
+
+#if defined(ECP_LOAD_GROUP) || defined(MBEDTLS_ECP_DP_CURVE25519_ENABLED) || \
+    defined(MBEDTLS_ECP_DP_CURVE448_ENABLED)
 /*
  * Create an MPI from embedded constants
  * (assumes len is an exact multiple of sizeof mbedtls_mpi_uint)
@@ -4513,7 +4515,9 @@
     X->n = len / sizeof(mbedtls_mpi_uint);
     X->p = (mbedtls_mpi_uint *) p;
 }
+#endif
 
+#if defined(ECP_LOAD_GROUP)
 /*
  * Set an MPI to static value 1
  */