msm: pcie: expand PCIe bandwidth scaling

Instead of supporting only GEN1 and GEN3 scaling, expand PCIe
bus driver to support scaling between GEN1, GEN2, and GEN3.
Also update PCIe bus driver to check the new target bandwidth
is met.

Change-Id: I0f42a350f3d20b64945f5bde1216868e5d4c8cb5
Signed-off-by: Tony Truong <truong@codeaurora.org>