MAINTAINERS: co-maintainance of KVM/{arm,arm64}

MAINTAINERS: co-maintainance of KVM/{arm,arm64}

The KVM/{arm,arm64} ports are sharing a lot of code, and are
effectively co-maintained (and have been for quite a while).

Make the situation official and list the two maintainers
for both ports.

Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Russell King <linux@arm.linux.org.uk>
Cc: Paolo Bonzini <pbonzini@redhat.com>
Cc: Gleb Natapov <gleb@redhat.com>
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>
Signed-off-by: Christoffer Dall <christoffer.dall@linaro.org>
Acked-by: Catalin Marinas <catalin.marinas@arm.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>