MAINTAINERS: Add Oliver Upton as co-maintainer of KVM/arm64

Going forward I intend to help Marc with maintaining KVM/arm64. We've
spoken about this quite a bit and he has been a tremendous help in
ramping up to the task (thank you!). We haven't worked out the exact
details of how the process will work, but the goal is to even out the
maintenance responsibilities to give us both ample time for development.

To that end, updating the maintainers entry to reflect the change.

Acked-by: Will Deacon <will@kernel.org>
Acked-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20230123210256.2728218-1-oliver.upton@linux.dev


Signed-off-by: Oliver Upton <oliver.upton@linux.dev>