arm: Use table walker clock that is inherited from CPU

This patch simplifies the scheduling of the next walk for the ARM
table walker. Previously it used the CPU clock, but as the table
walker inherits the clock from the CPU, it is cleaner to simply use
its own clock (which is the same).

src/arch/arm/table_walker.cc

@@ -797,7 +797,7 @@ void
 TableWalker::nextWalk(ThreadContext *tc)
 {
     if (pendingQueue.size())
-        schedule(doProcessEvent, tc->getCpuPtr()->clockEdge(Cycles(1)));
+        schedule(doProcessEvent, clockEdge(Cycles(1)));
 }