KVM: arm64: Allocate pages for hypervisor FF-A mailboxes

The FF-A proxy code needs to allocate its own buffer pair for
communication with EL3 and for forwarding calls from the host at EL1.

Reserve a couple of pages for this purpose and use them to initialise
the hypervisor's FF-A buffer structure.

Co-developed-by: Andrew Walbran <qwandor@google.com>
Signed-off-by: Andrew Walbran <qwandor@google.com>
Signed-off-by: Will Deacon <will@kernel.org>
Link: https://lore.kernel.org/r/20230523101828.7328-4-will@kernel.org


Signed-off-by: Oliver Upton <oliver.upton@linux.dev>