MIPS: Remove DMA_PERDEV_COHERENT

As now we are always managing DMA coherence on per dev bias,
there is no need to have such option. And it's not selected
by any platform.

Signed-off-by: Jiaxun Yang <jiaxun.yang@flygoat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>