KVM: arm64: Update MAINTAINERS to include selftests

As the KVM/arm64 selftests are routed via the kvmarm tree,
add the relevant references to the MAINTAINERS file.

Suggested-by: Andrew Jones <drjones@redhat.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20210622070732.zod7gaqhqo344vg6@gator