arm64: KVM: Skip HYP setup when already running in HYP

arm64: KVM: Skip HYP setup when already running in HYP

With the kernel running at EL2, there is no point trying to
configure page tables for HYP, as the kernel is already mapped.

Take this opportunity to refactor the whole init a bit, allowing
the various parts of the hypervisor bringup to be split across
multiple functions.

Reviewed-by: Christoffer Dall <christoffer.dall@linaro.org>
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>
(cherry picked from commit 1e947bad0b63b351cbdd9ad55ea5bf7e31c76036)
Signed-off-by: Alex Shi <alex.shi@linaro.org>