ARM: non-sec: reset CNTVOFF to zero

Before switching to non-secure, make sure that CNTVOFF is set
to zero on all CPUs. Otherwise, kernel running in non-secure
without HYP enabled (hence using virtual timers) may observe
timers that are not synchronized, effectively seeing time
going backward...

Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>
Acked-by: Ian Campbell <ijc@hellion.org.uk>