UPSTREAM: asm-generic/tlb: Guard with #ifdef CONFIG_MMU

The inner workings of the mmu_gather-based TLB invalidation mechanism
are not relevant to nommu configurations, so guard them with an #ifdef.
This allows us to implement future functions using static inlines
without breaking the build.

Acked-by: Nicholas Piggin <npiggin@gmail.com>
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Change-Id: I8d6673a8daa1ff4de448477b8f0bfc5cd0ec5719
Signed-off-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: Cyber Knight <cyberknight755@gmail.com>