usb: usb251xb: Add max power/current dts property support

This parameters may be varied in accordance with hardware specifics.
So lets add the corresponding settings to the usb251xb driver dts
specification.

Signed-off-by: Serge Semin <fancer.lancer@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>