At present this enables a few arch-specific members of the global_data
struct which are otherwise not part of the struct. As a result we have to
use #ifdef in various places.

The cost of always having these in the struct is small. Adjust things so
that we can use compile-time code instead of #ifdefs.
Signed-off-by: NSimon Glass <sjg@chromium.org>
Reviewed-by: NBin Meng <bmeng.cn@gmail.com>