soc: qcom: Introduce QMI encoder/decoder

Add the helper library for encoding and decoding QMI encoded messages.
The implementation is taken from lib/qmi_encdec.c of the Qualcomm kernel
(msm-3.18).

Modifications has been made to the public API, source buffers has been
made const and the debug-logging part was omitted, for now.

Acked-by: Chris Lew <clew@codeaurora.org>
Tested-by: Chris Lew <clew@codeaurora.org>
Tested-by: Srinivas Kandagatla <srinivas.kandagatla@linaro.org>
Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>
Signed-off-by: Andy Gross <andy.gross@linaro.org>