usb: usb3503: add PM functions

The usb3503 needs to switch to standby mode while suspending and should
switch to hub mode when resumed. Also we can control clock on PM
function.

Signed-off-by: Joonyoung Shim <jy0922.shim@samsung.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>