ARM: HYP/non-sec: move switch to non-sec to the last boot phase

Having the switch to non-secure in the "prep" phase is causing
all kind of troubles, as that stage can be called multiple times.

Instead, move the switch to non-secure to the last possible phase,
when there is no turning back anymore.

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