usb: dwc2: add shutdown callback to platform variant

In specific conditions (involving usb hubs) dwc2 devices can create a
lot of interrupts, even to the point of overwhelming devices running
at low frequencies. Some devices need to do special clock handling
at shutdown-time which may bring the system clock below the threshold
of being able to handle the dwc2 interrupts. Disabling dwc2-irqs
in a shutdown callbacks prevents reboots/poweroffs from getting stuck
in such cases.

The hsotg struct already contains an unused irq element, so we can
just use it to store the irq number for the shutdown callback.

Reviewed-by: Douglas Anderson <dianders@chromium.org>
Acked-by: John Youn <johnyoun@synopsys.com>
Signed-off-by: Heiko Stuebner <heiko.stuebner@collabora.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>