soc: qcom: smd: Support multiple channels per sdev

This patch allows chaining additional channels to a SMD device, enabling
implementation of multi-channel SMD devies - like Bluetooth.

Signed-off-by: Bjorn Andersson <bjorn.andersson@sonymobile.com>
Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>
Signed-off-by: Andy Gross <andy.gross@linaro.org>