efi: Mention that efi_info_get() is only used in the stub

This provides access to EFI tables after U-Boot has exited boot services.
It is not needed in the app since boot services remain alive and we can
just call them whenever needed.

Add a comment to explain this.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>