soc: qcom: smd: Introduce compile stubs

Introduce compile stubs for the SMD API, allowing consumers to be
compile tested.

Acked-by: Andy Gross <andy.gross@linaro.org>
Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>
Signed-off-by: David S. Miller <davem@davemloft.net>