ARM: nommu: Don't build smp_tlb.c for !CONFIG_MMU

Without an MMU we don't need to do any TLB maintenance. Until the addition
of 93dc6887 (ARM: 7684/1: errata: Workaround for Cortex-A15 erratum 798181
(TLBI/DSB operations)) building the tlb maintenance ops in smp_tlb.c worked,
though none of the contents were used.

Since that commit, however, SMP NOMMU has not been able to build. This patch
restores that ability by making the building of smp_tlb.c dependent on MMU.

Signed-off-by: Jonathan Austin <jonathan.austin@arm.com>
Acked-by: Catalin Marinas <catalin.marinas@arm.com>
CC: Will Deacon <will.deacon@arm.com>