Documentation/features: mark BATCHED_UNMAP_TLB_FLUSH doesn't apply to ARM64

BATCHED_UNMAP_TLB_FLUSH is used on x86 to do batched tlb shootdown by
sending one IPI to TLB flush all entries after unmapping pages rather
than sending an IPI to flush each individual entry.
On arm64, tlb shootdown is done by hardware. Flush instructions are
innershareable. The local flushes are limited to the boot (1 per CPU)
and when a task is getting a new ASID.
So marking this feature as "TODO" is not proper. ".." isn't good as
well. So this patch adds a "N/A" for this kind of features which are
not needed on some architectures.

Signed-off-by: Barry Song <[e-mail]>
Acked-by: Will Deacon <[e-mail]>
Cc: Mel Gorman <[e-mail]>
Cc: Andy Lutomirski <[e-mail]>
Cc: Catalin Marinas <[e-mail]>
Cc: Will Deacon <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Jonathan Corbet <[e-mail]>

Documentation/features/arch-support.txt

···
     | ok |  # feature supported by the architecture
     |TODO|  # feature not yet supported by the architecture
     | .. |  # feature cannot be supported by the hardware
+    | N/A|  # feature doesn't apply to the architecture
 

Documentation/features/vm/TLB/arch-support.txt

···
     |       alpha: | TODO |
     |         arc: | TODO |
     |         arm: | TODO |
-    |       arm64: | TODO |
+    |       arm64: | N/A  |
     |        csky: | TODO |
     |       h8300: |  ..  |
     |     hexagon: | TODO |