microblaze: Fix tlb_skip variable on noMMU system

TLBs are available only for MMU systems.

Error log:
arch/microblaze/kernel/setup.c: In function 'debugfs_tlb':
arch/microblaze/kernel/setup.c:217: error: 'tlb_skip' undeclared (first use in this function)
arch/microblaze/kernel/setup.c:217: error: (Each undeclared identifier is reported only once
arch/microblaze/kernel/setup.c:217: error: for each function it appears in.)
make[1]: *** [arch/microblaze/kernel/setup.o] Error 1

Signed-off-by: Michal Simek <monstr@monstr.eu>

arch/microblaze/kernel/setup.c

@@ -209,6 +209,7 @@ static int microblaze_debugfs_init(void)
 }
 arch_initcall(microblaze_debugfs_init);
 
+# ifdef CONFIG_MMU
 static int __init debugfs_tlb(void)
 {
 	struct dentry *d;
@@ -221,6 +222,7 @@ static int __init debugfs_tlb(void)
 		return -ENOMEM;
 }
 device_initcall(debugfs_tlb);
+# endif
 #endif
 
 static int dflt_bus_notify(struct notifier_block *nb,