msm: ADSPRPC: Set the rpc mode via ioctl.

Set the rpc mode to either serial or parallel via an ioctl, and ensure
that output buffers are invalidated in serial mode before calling into
the dsp.

Change-Id: I565faba24106cd7e3bba5368b06c4036bb468483
Acked-by: Anatoly Yakovenko <anatolyy@qti.qualcomm.com>
Signed-off-by: Mitchel Humpherys <mitchelh@codeaurora.org>