KVM: arm/arm64: Update MAINTAINERS entries

For historical reasons, KVM/arm and KVM/arm64 have had different
entries in the MAINTAINER file. This makes little sense, as they are
maintained together.

On top of that, we have a bunch of talented people helping with
the reviewing, and they deserve to be mentioned in the consolidated
entry.

Acked-by: Christoffer Dall <christoffer.dall@arm.com>
Acked-by: Suzuki K Poulose <suzuki.poulose@arm.com>
Acked-by: James Morse <james.morse@arm.com>
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>