MIPS: only select ARCH_HAS_UNCACHED_SEGMENT for non-coherent platforms

While mips might architecturally have the uncached segment all the time,
the infrastructure to use it is only need on platforms where DMA is
at least partially incoherent.  Only select it for those configuration
to fix a build failure as the arch_dma_prep_coherent symbol is also only
provided for non-coherent platforms.

Fixes: 2e96e04d ("MIPS: use the generic uncached segment support in dma-direct")
Reported-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Acked-by: Paul Burton <paul.burton@mips.com>
Tested-by: Guenter Roeck <linux@roeck-us.net>