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 <[e-mail]>
Cc: Bin Meng <[e-mail]>

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