defconfig: 8974: Enable ADSP RPC driver

The ADSP RPC driver allows for clients to make remote method
invocations across processor boundary to DSP processor. Enable
this driver for MSM8974.

Change-Id: I525d8496955329aa576b75a462e0f500b6223b14
Acked-by: Sathish Ambley <sambley@qti.qualcomm.com>
Signed-off-by: Mitchel Humpherys <mitchelh@codeaurora.org>