mpc85xx: remove redudant code with lib_ppc/interrupts.c

For some reason we duplicated the majority of code in lib_ppc/interrupts.c
not show how that happened, but there is no good reason for it.

Use the interrupt_init_cpu() and timer_interrupt_cpu() since its why
they exist.

Signed-off-by: Kumar Gala <galak@kernel.crashing.org>