usb: xhci: add the suspend/resume functionality

Adds power management support to xHCI platform driver.

This patch facilitates the transition of xHCI host controller
between S0 and S3/S4 power states, during suspend/resume cycles.

Signed-off-by: Abhilash Kesavan <a.kesavan@samsung.com>
Signed-off-by: Vikas C Sajjan <vikas.sajjan@linaro.org>
CC: Doug Anderson <dianders@chromium.org>
Signed-off-by: Felipe Balbi <balbi@ti.com>
Signed-off-by: Sarah Sharp <sarah.a.sharp@linux.intel.com>