drivers: Add dma removed ops

The current DMA coherent pool assumes that there is a kernel
mapping at all times for hte entire pool. This may not be
what we want for the entire times. Add the dma_removed ops to
support this use case.

Change-Id: Ie4f1e9bdf57b79699fa8fa7e7a6087e6d88ebbfa
Signed-off-by: Laura Abbott <lauraa@codeaurora.org>