msm: pcie: support dynamic CX and rate change clock scaling

By default, PCIe driver will vote CX and rate change clock to
support the highest GEN speed. Not all devices support and link
up with RC max GEN speed. To save power, PCIe will scale down
the CX corner and rate change clock based on the actual link up
GEN speed.

Change-Id: I9543d1cb6e707686553a159ce3b03edaa85eb39c
Signed-off-by: Tony Truong <truong@codeaurora.org>