MAINTAINERS: Update KVM RISC-V entry to cover selftests support

We update KVM RISC-V maintainers entry to include appropriate KVM
selftests directories so that RISC-V related KVM selftests patches
are CC'ed to KVM RISC-V mailing list.

Signed-off-by: Anup Patel <anup@brainfault.org>