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>