arm: mm: dma-mapping: fix unused function warning

Compile the static function is_dma_coherent, when config
ARM_DMA_USE_IOMMU is selected. Compiler's configured to
treat unused function as error will pass through.

Change-Id: I0ad343ce9c7a50faf8c2085af0c60d709b07559d
Signed-off-by: Rakesh Kumar <rakekuma@codeaurora.org>
Signed-off-by: Sivasri Kumar Vanka <sivasri@codeaurora.org>