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.

Change-Id: I54a74978d2ae30e90b3ee67eba0912b9f51ca935
Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>
Patch-mainline: linux-arm-msm @ 15/11/2017 12:10
[clew@codeaurora.org: Merge differences between v1 and v3 patchsets
 from "soc: qcom: Introduce QMI encoder/decoder"]
Signed-off-by: Chris Lew <clew@codeaurora.org>