usb: dwc3: of-simple: set dev_pm_ops

dwc3_of_simple_dev_pm_ops has never been used since the initial support
by commit 16adc674 ("usb: dwc3: add generic OF glue layer").

I guess it just missed to set .pm struct member.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>