rpmsg: Introduce Qualcomm SMD backend

This introduces a new rpmsg backend for the Qualcomm SMD system,
allowing communication with various remote processors found in Qualcomm
platforms. The implementation is based on, and intends to replace,
drivers/soc/qcom/smd.c with the necessary adaptions for fitting with the
rpmsg core.

Based on original work by Sricharan R <sricharan@codeaurora.org>

Cc: Sricharan R <sricharan@codeaurora.org>
Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>