m32r: fix build failure due to SMP and MMU

One of the randconfig build failed with the error:

  arch/m32r/kernel/smp.c: In function 'smp_flush_tlb_mm':
  arch/m32r/kernel/smp.c:283:20: error: subscripted value is neither array nor pointer nor vector
    mmc = &mm->context[cpu_id];
                      ^
  arch/m32r/kernel/smp.c: In function 'smp_flush_tlb_page':
  arch/m32r/kernel/smp.c:353:20: error: subscripted value is neither array nor pointer nor vector
    mmc = &mm->context[cpu_id];
                      ^
  arch/m32r/kernel/smp.c: In function 'smp_invalidate_interrupt':
  arch/m32r/kernel/smp.c:479:41: error: subscripted value is neither array nor pointer nor vector
    unsigned long *mmc = &flush_mm->context[cpu_id];

It turned out that CONFIG_SMP was defined but CONFIG_MMU was not
defined.  But arch/m32r/include/asm/mmu.h only defines mm_context_t as
an array when both CONFIG_SMP and CONFIG_MMU are defined.  And
arch/m32r/kernel/smp.c is always using context as an array.  So without
MMU SMP can not work.
Signed-off-by: Sudip Mukherjee <sudip@vectorindia.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

arch/m32r/Kconfig

@@ -276,6 +276,7 @@ source "kernel/Kconfig.preempt"
 
 config SMP
 	bool "Symmetric multi-processing support"
+	depends on MMU
 	---help---
 	  This enables support for systems with more than one CPU. If you have
 	  a system with only one CPU, say N. If you have a system with more