msm: rtb: Convert away from allocate_contiguous_ebi APIs

The allocate_contiguous_ebi APIs are now deprecated. Move
to using dma_alloc_coherent for the RTB buffer. Because we
aren't using allocate_contiguous_ebi for allocation, change
the devicetree bindings as well to support using CMA or just
a regular size from the generic region.

Change-Id: I8ab0b7336924a78b40c68af3f215d7bfd2fba33c
Signed-off-by: Laura Abbott <lauraa@codeaurora.org>