ARM: HYP/non-sec: migrate stack

The current code that switches into HYP mode doesn't bother to set
up a stack for HYP mode.  This doesn't work for EFI applications
as they expect a usable stack.  Fix this by migrating the stack
pointer from SP_svc to SP_hyp while in Monitor mode.
This restores the stack pointer when we drop into HYP mode.

Signed-off-by: Mark Kettenis <kettenis@openbsd.org>
Signed-off-by: Alexander Graf <agraf@suse.de>