[mbx] Add BUILD file

Signed-off-by: Amaury Pouly <amaury.pouly@lowrisc.org>

hw/ip/mbx/BUILD

@@ -0,0 +1,10 @@
+# Copyright lowRISC contributors (OpenTitan project).
+# Licensed under the Apache License, Version 2.0, see LICENSE for details.
+# SPDX-License-Identifier: Apache-2.0
+
+package(default_visibility = ["//visibility:public"])
+
+filegroup(
+    name = "all_files",
+    srcs = glob(["**"]),
+)