efi: Include device-path functions in the EFI API docs

efi: Include device-path functions in the EFI API docs

Include these function so they can be browsed in the API docs. Exclude
END since it causes a warning, which becomes an error:

   ./include/efi_device_path.h:22: warning: cannot understand function
      prototype: 'const struct efi_device_path END; '

Signed-off-by: Simon Glass <sjg@chromium.org>
Suggested-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>