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>

MAINTAINERS

@@ -241,6 +241,7 @@ F: disas/ppc.c
 RISC-V
 M: Michael Clark <mjc@sifive.com>
 M: Palmer Dabbelt <palmer@sifive.com>
+M: Alistair Francis <Alistair.Francis@wdc.com>
 M: Sagar Karandikar <sagark@eecs.berkeley.edu>
 M: Bastian Koppelmann <kbastian@mail.uni-paderborn.de>
 S: Maintained