msm: pcie: provide APIs to prevent and allow PCIe ASPM L1

Provide clients an API to exit and prevent the link from active
state power management (ASPM) L1.  Also, give clients the
control to allow the link to enter ASPM L1.

Change-Id: Iedabb4eed51adb981f0eb1f204b468e7b7fbc678
Signed-off-by: Tony Truong <truong@codeaurora.org>