efi_loader: Add runtime services

After booting has finished, EFI allows firmware to still interact with the OS
using the "runtime services". These callbacks live in a separate address space,
since they are available long after U-Boot has been overwritten by the OS.

This patch adds enough framework for arbitrary code inside of U-Boot to become
a runtime service with the right section attributes set. For now, we don't make
use of it yet though.

We could maybe in the future map U-boot environment variables to EFI variables
here.

Signed-off-by: Alexander Graf <agraf@suse.de>
Reviewed-by: Simon Glass <sjg@chromium.org>
Tested-by: Simon Glass <sjg@chromium.org>