usb: usb251xb: dt: add unit suffix to oc-delay and power-on-time

Rename oc-delay-* to oc-delay-us and make it expect a time value.
Furthermore add -ms suffix to power-on-time. There changes were
suggested by Rob Herring in https://lkml.org/lkml/2017/2/15/1283

.

Signed-off-by: Richard Leitner <richard.leitner@skidata.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>