soc: qcom: documentation: Update SMD/RPM Docs

This patch moves the qcom,smd-rpm.txt to the correct location and splits
out the smd and rpm documentation.  In addition, a smd-rpm-regulator
document is added.

Signed-off-by: Andy Gross <agross@codeaurora.org>
Acked-by: Bjorn Andersson <bjorn.andersson@sonymobile.com>
Signed-off-by: Mark Brown <broonie@kernel.org>