ARM: kill off arch_is_coherent

With ixp2xxx removed, there are no platforms that define arch_is_coherent,
so the last occurrences of arch_is_coherent can be removed. Any new
platform with coherent i/o should use coherent dma mapping functions.

Signed-off-by: Rob Herring <rob.herring@calxeda.com>
Cc: Russell King <linux@arm.linux.org.uk>
Cc: Marek Szyprowski <m.szyprowski@samsung.com>
Signed-off-by: Marek Szyprowski <m.szyprowski@samsung.com>