sh: stub __flush_tlb_global() definition for nommu.

This fixes up the nommu build with a stub definition for
__flush_tlb_global(), now used by the reboot code.
Signed-off-by: NPaul Mundt <lethal@linux-sh.org>

arch/sh/mm/nommu.c

@@ -67,6 +67,10 @@ void local_flush_tlb_kernel_range(unsigned long start, unsigned long end)
 	BUG();
 }
 
+void __flush_tlb_global(void)
+{
+}
+
 void __update_tlb(struct vm_area_struct *vma, unsigned long address, pte_t pte)
 {
 }