msm: ADSPRPC: Allocate cached memory for ADSP RPC buffers

Pass ION_FLAG_CACHED flag to allocate cached memory for ADSP
RPC buffers that are flushed before being passed to the remote
processor.

Change-Id: I5faa2c074157efcdc3d859090c046b6a700e52a4
Acked-by: Sathish Ambley <sambley@qti.qualcomm.com>
Signed-off-by: Mitchel Humpherys <mitchelh@codeaurora.org>