efi_loader: ARM: don't attempt to enter non-secure mode twice

Multiple EFI binaries may be executed in sequence.  So if we already
are in non-secure mode after running the first one we should skip
the switching code since it no longer works once we're non-secure.

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