dt-bindings: clock: sifive: add MIT license as an option for the header file

At Bin Meng's request, add the MIT license as an option for the SiFive
FU540 PRCI header file.
Signed-off-by: Paul Walmsley <paul.walmsley@sifive.com>
Cc: Bin Meng <bmeng.cn@gmail.com>

include/dt-bindings/clock/sifive-fu540-prci.h

-/* SPDX-License-Identifier: GPL-2.0 */
+/* SPDX-License-Identifier: (GPL-2.0 OR MIT) */
 /*
  * Copyright (C) 2018-2019 SiFive, Inc.
  * Wesley Terpstra