Update SUMMARY.md

SUMMARY.md

@@ -17,6 +17,7 @@
     * [Fixmaps and ioremap](mm/linux-mm-2.md)
 * [Interrupts]()
 * [vsyscalls and vdso]()
+* [SMP]()
 * [Concepts](Concepts/README.md)
     * [Per-CPU variables](Concepts/per-cpu.md)
     * [Cpumasks](Concepts/cpumask.md)