KVM: arm64: nVHE: Migrate hyp-init to SMCCC

To complete the transition to SMCCC, the hyp initialization is given a
function ID. This looks neater than comparing the hyp stub function IDs
to the page table physical address.

Some care is taken to only clobber x0-3 before the host context is saved
as only those registers can be clobbered accoring to SMCCC. Fortunately,
only a few acrobatics are needed. The possible new tpidr_el2 is moved to
the argument in x2 so that it can be stashed in tpidr_el2 early to free
up a scratch register. The page table configuration then makes use of
x0-2.

Signed-off-by: Andrew Scull <ascull@google.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20200915104643.2543892-19-ascull@google.com