usb: dwc3-msm: Allow PM suspend in host mode irrespective of runtimePM

DWC3 driver uses wakeup_source and allows pm_suspend only after
entering low power mode as part of runtime suspend. This prevents
PM suspend in host mode if connected device's driver doesn't
support runtime or selective suspend.
Add support in driver where user can specify using dtsi attribute
to not use wakeup source in host mode. It will allow system to enter
deep sleep or pm_suspend irrespective of runtime PM state of XHCI.
On high level below are the changes in default behavior:
 -Do not use wakeup_source for DWC3 when operating in host mode.
 -For host mode, devices will suspend upon PM suspend and resumed
  after PM resume.

Change-Id: Ia1d450e6dd99e094e48caf9723a355b7870b54dd
Signed-off-by: Ajay Agarwal <ajaya@codeaurora.org>