msm: pcie: provide client the ability to control PCIe target_link_speed

There are usecases where clients want to change the maximum GEN
speed that PCIe can link up with. Provide clients the ability to
control the maximum PCIe GEN speed to for such usecases.

Change-Id: I834bee2063133546c9b0ebac2d5328ffbc5ceac6
Signed-off-by: Tony Truong <truong@codeaurora.org>