iommu/iommu-debug: Add validation support for per-buffer coherent mappings

Added new debugfs files/nodes to perform basic validation of the
per-buffer IO coherent dma-mapping that was recently implemented.
Also, added a node to print out the PTE for a given IOVA.

Nodes added:
dma_attach
dma_map
dma_unmap
dma_atos
virt_addr
pte

Change-Id: I041ab5f4e37e50dd3f58e2def3fdd3125a60629c
Signed-off-by: Sudarshan Rajagopalan <sudaraja@codeaurora.org>