[POWERPC] PS3: Vuart cleanups

Cleanups for the PS3 vuart driver.

- Hide driver private data from external interface with new structure
  ps3_vuart_port_priv.
- Fix masking bug in ps3_vuart_get_interrupt_status().
- Add new helper routine ps3_vuart_clear_rx_bytes() to flush rx buffer.
- Add new variable probe_mutex to serialize probe and destroy routines.
- Rename some symbols.
- Add platform check in ps3_vuart_bus_init().

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