powerpc: Add a shutdown member to vio_driver

Add a shutdown member to struct vio_driver. We also need vio_bus_shutdown()
which converts from struct device to struct vio_dev and knows how to extract
the struct vio_driver.

Original patch adjusted for different location of vio.c.

Signed-off-by: Michael Ellerman <michael@ellerman.id.au>
Signed-off-by: Stephen Rothwell <sfr@canb.auug.org.au>