kvmtool/arm: Fix timer trigger

KVM exposes a level triggered timer to the guest, and yet kvmtool
presents it as being edge-triggered in the DT. Let's fix it and
match what the kernel exposes.

Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>