dma-mapping: Remove exposure of dma_get_[base/size]

Now that dma_get_[base/size] are not needed by other
drivers, remove the implementation of dma_get_size(),
and limit the scope of dma_get_base() to its original
visibility.

Change-Id: I38d6024b43608743217ebfd44a35ccb4da714386
Signed-off-by: Isaac J. Manjarres <isaacm@codeaurora.org>