common: Add "ifndef __ASSEMBLY__" in asm/global_data.h

Commit "common: Drop asm/global_data.h from common header" added
asm/global_data.h into secure.h. However, secure.h will be included
by psci.S. Adding asm/global_data.h has caused compilation failure in
pcsi.S. Add "ifndef __ASSEMBLY__" in asm/global_data.h.

Signed-off-by: Siew Chin Lim <elly.siew.chin.lim@intel.com>
Reviewed-by: Simon Glass <sjg@chromium.org>