ARM: v6k: DMA_CACHE_RWFO isn't appropriate for non-v6k CPUs

Limit DMA_CACHE_RWFO to only v6k SMP CPUs - V6 CPUs aren't SMP capable,
so the read/write for ownership work-around doesn't apply to them.

Acked-by: Will Deacon <will.deacon@arm.com>
Tested-by: Sourav Poddar <sourav.poddar@ti.com>
Tested-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>