ARM: virt: boot secondary CPUs through the right entry point

Secondary CPUs should use the __hyp_stub_install_secondary entry
point, so boot mode inconsistencies can be detected.

Cc: <stable@vger.kernel.org>
Acked-by: Dave Martin <dave.martin@linaro.org>
Reported-by: Ian Molton <ian.molton@collabora.co.uk>
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>