efi_loader: ARM: run EFI payloads non-secure

If desired (and possible) switch into HYP mode or non-secure SVC mode
before calling the entry point of an EFI application.  This allows
U-Boot to provide a usable PSCI implementation and makes it possible
to boot kernels into hypervisor mode using an EFI bootloader.

Based on diffs from Heinrich Schuchardt and Alexander Graf.

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