msm: ep-pcie: Add support to vote and scale MX rail

Chipsets which have PCIe Gen4 PHY and controller on MX rail
requires explicit vote for higher corners than the default.
Therefore add support to vote for and scale MX rail based on the
Gen speed which PCIe link operates at.

Change-Id: I6aa595aa6e3955ff09d8eeae604431b47a14985a
Signed-off-by: Gauri Joshi <gaurjosh@codeaurora.org>