USB: xhci: Fix compile error when CONFIG_PM=n

Fix this error when CONFIG_PM is not enabled:

drivers/usb/host/xhci.c:675: error: implicit declaration of function 'usb_root_hub_lost_power'

Wrap xhci_suspend() and xhci_resume() into an ifdef CONFIG_PM, along with
the functions that only they call -- xhci_save_registers() and
xhci_restore_registers().

Reported-by: Randy Dunlap <randy.dunlap@oracle.com>
Signed-off-by: Sarah Sharp <sarah.a.sharp@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>