common: add dma_mmap_from_coherent() function

Add a common helper for dma-mapping core for mapping a coherent buffer
to userspace.

Change-Id: I9b67d53b50f4761455ff66634d65e6510faa62de
Reported-by: Subash Patel <subashrp@gmail.com>
Signed-off-by: Marek Szyprowski <m.szyprowski@samsung.com>
Acked-by: Kyungmin Park <kyungmin.park@samsung.com>
Tested-By: Subash Patel <subash.ramaswamy@linaro.org>
Signed-off-by: Laura Abbott <lauraa@codeaurora.org>