dma-mapping: Add dma_remap functions

After getting an allocation from dma_alloc_coherent, there
may be cases where it is neccessary to remap the handle
into the CPU's address space (e.g. no CPU side mapping was
requested at allocation time but now one is needed). Add
APIs to bring a handle into the CPU address space again.

Signed-off-by: Laura Abbott <lauraa@codeaurora.org>
[imaund@codeaurora.org: resolved context conflicts and added support
  for remap 'no_warn' argument]
Signed-off-by: Ian Maund <imaund@codeaurora.org>