dma-mapping: fix build when !CONFIG_ARM_DMA_USE_IOMMU

Allow compilation when CONFIG_ARM_DMA_USE_IOMMU is not
selected by providing necessary stub functions.

Change-Id: I04e3aa63407064e8d9c9550a5cb0a82ede899f00
Signed-off-by: Shiraz Hashim <shashim@codeaurora.org>
Signed-off-by: Patrick Daly <pdaly@codeaurora.org>