ssl_tls: fix guards for accelerated ECDH

Signed-off-by: Valerio Setti <valerio.setti@nordicsemi.no>
diff --git a/include/mbedtls/pk.h b/include/mbedtls/pk.h
index 3de7a8f..f029265 100644
--- a/include/mbedtls/pk.h
+++ b/include/mbedtls/pk.h
@@ -193,6 +193,12 @@
 #endif /* PSA_WANT_ALG_ECDSA */
 #endif /* MBEDTLS_USE_PSA_CRYPTO */
 
+/* Symbol for ECDH capabilities, no matter how it is provided */
+#if (defined(MBEDTLS_USE_PSA_CRYPTO) && defined(PSA_HAVE_FULL_ECDH)) || \
+    (!defined(MBEDTLS_USE_PSA_CRYPTO) && defined(MBEDTLS_ECDH_C))
+#define MBEDTLS_PK_CAN_ECDH
+#endif
+
 #if defined(MBEDTLS_PK_CAN_ECDSA_VERIFY) || defined(MBEDTLS_PK_CAN_ECDSA_SIGN)
 #define MBEDTLS_PK_CAN_ECDSA_SOME
 #endif