dt-bindings: riscv,cpu-intc: Cleanups from a missed review

I managed to miss one of Rob's code reviews on the mailing list
<http://lists.infradead.org/pipermail/linux-riscv/2018-August/001139.html>.
The patch has already been merged, so I'm submitting a fixup.

Sorry!

Fixes: b67bc7cb

 ("dt-bindings: interrupt-controller: RISC-V local interrupt controller")
Cc: Rob Herring <robh@kernel.org>
Cc: Christoph Hellwig <hch@infradead.org>
Cc: Karsten Merker <merker@debian.org>
Signed-off-by: Palmer Dabbelt <palmer@sifive.com>