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>