mpc83xx: immap_83xx: add spi8xxx_t in immap for mpc8309

Allow drivers/spi/mpc8xxx_spi.c to be built for an mpc8309 target.

Signed-off-by: Rasmus Villemoes <rasmus.villemoes@prevas.dk>
Reviewed-by: Mario Six <mario.six@gdsys.cc>