dma-coherent: Expose device base address and size

Allow clients to be able to determine the device base address and
size of a dma coherent memory region.

This is required for clients that want to explicitly manage that memory.

Change-Id: I3e5b721d9d318526422ec26995cc1af7b7f04077
Signed-off-by: Liam Mark <lmark@codeaurora.org>