powerpc/ps3: mark ps3_system_bus_type static

ps3_system_bus_type is only used inside of system-bus.c, so remove
the external declaration and the very outdated comment next to it.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Acked-by: Geoff Levand <geoff@infradead.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20221122072225.423432-1-hch@lst.de