x86: reserve SLIT

save the SLIT, in case we are using fixmap to read it, and that fixmap
could be cleared by others.

Signed-off-by: Yinghai Lu <yhlu.kernel@gmail.com>
Cc: Suresh Siddha <suresh.b.siddha@intel.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>