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>