usb: xhci-plat: Remove system PM call backs

system PM callbacks are just calling xhci_suspend() and
xhci_resume() functions which are already called from
runtime PM callbacks. Calling it from system PM again
results into un-clocked register access.

Change-Id: I4e3216773246e5f4ec94d8e921b98c51fff2654d
Signed-off-by: Hemant Kumar <hemantk@codeaurora.org>