spl: Add a comment to spl_set_bd()

There is a strange feature to set global_data to a data-section variable
early in SPL. This only works if SPL actually has access to SRAM which is
not the case on x86, for eaxmple. Add a comment to this effect.

Reviewed-by: Simon Glass <sjg@chromium.org>
Signed-off-by: Simon Glass <sjg@chromium.org>