usb: dwc3 / xhci_plat: Call xhci_suspend/resume when entering/exiting LPM

Before placing the controller in low power mode using runtime
suspend, if host mode is active, xhci_suspend() should be
called to properly save state, stop root hub timers and clear
the Run/Stop bit. Likewise, if the controller receives an event
not handled by the HCD IRQ while in host suspend, it should resume
the host driver accordingly.

Add calls to xhci_suspend/resume by introducing runtime PM callbacks
to the xhci_plat driver (which were previously removed in commit
9db42d39 "usb: dwc3_otg: Remove phy dependency from xhci-plat").

CRs-Fixed: 590596
Change-Id: Ifc8016d22d2ad3f5dcd51881d22846952d37667e
Signed-off-by: Jack Pham <jackp@codeaurora.org>