usb: dwc3: xilinx: add shutdown callback

Adds a shutdown callback to ensure that the XHCI stack is properly
shutdown in reboot/shutdown path.

In kexec flow, kernel_restart_prepare() performs actions necessary
to prepare the system for a restart and invokes device_shutdown. To
ensure proper shutdown attach the dwc3 shutdown implementation which
mirrors the remove method.

$ kexec -e

<snip>
xhci-hcd xhci-hcd.0.auto: remove, state 1
usb usb1: USB disconnect, device number 1
usb 1-1: USB disconnect, device number 6
xhci-hcd xhci-hcd.0.auto: USB bus 1 deregistered
kexec_core: Starting new kernel

Signed-off-by: Radhey Shyam Pandey <radhey.shyam.pandey@amd.com>
Acked-by: Thinh Nguyen <Thinh.Nguyen@synopsys.com>
Link: https://lore.kernel.org/r/1748977771-714153-1-git-send-email-radhey.shyam.pandey@amd.com


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