Add Alistair as a RISC-V Maintainer

Alistair has been contributing to the RISC-V QEMU port for a while now
so I'd like him to be officially listed as a maintainer.  I've checked
with the other RISC-V maintainers and there are no objections, and I've
also checked with Alistair so he knows I'm volunteering him.

Signed-off-by: Palmer Dabbelt <palmer@sifive.com>
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