arch: powerpc: ps3: Make ps3_system_bus_type const

arch: powerpc: ps3: Make ps3_system_bus_type const

Because driver core can properly handle constant struct bus_type,
move the ps3_system_bus_type to be a constant structure as well,
placing it into read-only memory which can not be modified at runtime.

Signed-off-by: Adrian Barnaś <abarnas@google.com>
Reviewed-by: Geert Uytterhoeven <geert@linux-m68k.org>
Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Acked-by: Geoff Levand <geoff@infradead.org>
Link: https://patch.msgid.link/20250918114840.53581-1-abarnas@google.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>