[POWERPC] PS3: Vuart rework

PS3 vuart updates to reflect the new PS3 unified device support.
 - Move vuart devices to the PS3 system bus.
 - Replace use of ps3_vuart_port_device with ps3_system_bus_device.
 - Make the PS3 vuart bus driver a loadable module.
 - Add remove() and shutdown() routines.
 - Move ps3_vuart_work into ps3_vuart_port_priv.tx_list.
 - Remove redundant spinlock ps3_vuart_work.lock.
 - No longer free ps3_vuart_port_device.priv on shutdown.
 - Cleanup Kconfig defs.
 - Export symbols needed for modular port drivers.
 - Arrange to use port numbers found in repository.
 - Fix bugs in ps3_vuart_read_async() and polled reading
 - Cleanup handling of shared interrupt with ps3_vuart_bus_interrupt_get()
   and ps3_vuart_bus_interrupt_put()
 - Add more comments to vuart.c.

Signed-off-by: Geoff Levand <geoffrey.levand@am.sony.com>
Signed-off-by: Paul Mackerras <paulus@samba.org>