Curve448 is not yet supported via the PSA API

Filed as https://github.com/ARMmbed/mbedtls/issues/4249. In the
meantime, disable the feature.

Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>
diff --git a/include/mbedtls/config_psa.h b/include/mbedtls/config_psa.h
index ea82280..c46ed56 100644
--- a/include/mbedtls/config_psa.h
+++ b/include/mbedtls/config_psa.h
@@ -642,7 +642,8 @@
 #define PSA_WANT_ECC_MONTGOMERY_255
 #endif
 
-#if defined(MBEDTLS_ECP_DP_CURVE448_ENABLED)
+/* Curve448 is not yet supported via the PSA API (https://github.com/ARMmbed/mbedtls/issues/4249) */
+#if 0 && defined(MBEDTLS_ECP_DP_CURVE448_ENABLED)
 #define MBEDTLS_PSA_BUILTIN_ECC_MONTGOMERY_448 1
 #define PSA_WANT_ECC_MONTGOMERY_448
 #endif