powerpc/83xx: consolidate init_IRQ functions

On mpc83xx platform nearly all _init_IRQ functions look alike. They either
just setup ipic, or setup ipic and QE PIC. Separate this to special functions
to be either referenced from ppc_md, or called from board file.

Signed-off-by: Dmitry Eremin-Solenikov <dbaryshkov@gmail.com>
Signed-off-by: Kumar Gala <galak@kernel.crashing.org>