msm: adsprpc: skip CPU cache synchronization for non-coherent buffers

Skip synchronization of CPU cache for fastrpc buffers registered
as non-coherent, by DMA mapping them with the appropriate attribute
as the fastrpc driver will take care of cache maintenance.

Change-Id: I1e067270345eaa320ede76fc12ec450c341ca87a
Acked-by: Thyagarajan Venkatanarayanan <venkatan@qti.qualcomm.com>
Signed-off-by: Tharun Kumar Merugu <mtharu@codeaurora.org>