arm: mm: Fix compilation error when IOMMU is disabled

Fix compilation when DMA uses IOMMU with the config flag
CONFIG_ARM_DMA_USE_IOMMU.

Change-Id: I6f20dbca6f3090a7d0d5d720800a12feed5d0dc2
Signed-off-by: Jeevan Shriram <jshriram@codeaurora.org>