msm: 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: I1c2896d083fd57e02e1b19f7a75f356129d0e40f
Signed-off-by: Tony Truong <truong@codeaurora.org>