usb: usb251xb: add vdd supply support

Currently we don't handle the supply. We need to add the supply support
to be able to switch the supply off e.g. during a suspend-to-ram
operation. So we can guarantee a correct (re-)initialization.

Signed-off-by: Marco Felsch <m.felsch@pengutronix.de>
Reviewed-by: Richard Leitner <richard.leitner@skidata.com>
Tested-by: Richard Leitner <richard.leitner@skidata.com>
Link: https://lore.kernel.org/r/20190917144449.32739-3-m.felsch@pengutronix.de


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>