xhci: plat: Register shutdown for xhci_plat

Shutdown should be called for xhci_plat devices especially for
situations where kexec might be used by stopping DMA
transactions.

Signed-off-by: Adam Wallis <awallis@codeaurora.org>
Signed-off-by: Mathias Nyman <mathias.nyman@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>