ARM: dts: msm: add MX entries to qcom,bw-scale for PCIe on shima

Explicit MX voting and scaling is required for PCIe on some
chipsets and there are updates to PCIe bus driver to support
this. Although PCIe on shima does not need to explicitly vote for
MX, updates to shima PCIe DT node are required to be compatible
with the new qcom,bw-scale structure. Therefore, add MX entries
to qcom,bw-scale for PCIe on shima.

Change-Id: I4b93b92ee8e401c554e17e769432d3c22998ad98