powerpc/kexec: Fix mismatched ifdefs for PPC64/SMP.

Commit b3df895a "powerpc/kexec: Add support for FSL-BookE"
introduced the original PPC_STD_MMU_64 checks around the function
crash_kexec_wait_realmode().   Then commit c2be0548
"powerpc: Fix default_machine_crash_shutdown #ifdef botch" changed
the ifdef around the calling site to add a check on SMP, but the
ifdef around the function itself was left unchanged, leaving an
unused function for PPC_STD_MMU_64=y and SMP=n

Rather than have two ifdefs that can get out of sync like this,
simply put the corrected conditional around the function and use
a stub to get rid of one set of ifdefs completely.
Signed-off-by: Paul Gortmaker <paul.gortmaker@windriver.com>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>

arch/powerpc/kernel/crash.c

@@ -163,7 +163,7 @@ static void crash_kexec_prepare_cpus(int cpu)
 }
 
 /* wait for all the CPUs to hit real mode but timeout if they don't come in */
-#ifdef CONFIG_PPC_STD_MMU_64
+#if defined(CONFIG_PPC_STD_MMU_64) && defined(CONFIG_SMP)
 static void crash_kexec_wait_realmode(int cpu)
 {
 	unsigned int msecs;
@@ -188,6 +188,8 @@ static void crash_kexec_wait_realmode(int cpu)
 	}
 	mb();
 }
+#else
+static inline void crash_kexec_wait_realmode(int cpu) {}
 #endif
 
 /*
@@ -344,9 +346,7 @@ void default_machine_crash_shutdown(struct pt_regs *regs)
 	crash_save_cpu(regs, crashing_cpu);
 	crash_kexec_prepare_cpus(crashing_cpu);
 	cpu_set(crashing_cpu, cpus_in_crash);
-#if defined(CONFIG_PPC_STD_MMU_64) && defined(CONFIG_SMP)
 	crash_kexec_wait_realmode(crashing_cpu);
-#endif
 
 	machine_kexec_mask_interrupts();