powerpc/mpic: Add in-core support for cascaded MPICs

The Cell and PowerMac platforms use virtually identical cascaded-IRQ
setup code, so just merge it into the core.  Ideally this code would
trigger automatically when an MPIC device-node specifies an "interrupts"
property, perhaps even enabling MPIC_SECONDARY along the way.

Unfortunately, Benjamin Herrenschmidt has had bad experiences in the
past with the quality of Apple PowerMac device-trees, so to be safe we
will only try to parse out an IRQ if the MPIC_SECONDARY flag is set by
the caller.

Signed-off-by: Kyle Moffett <Kyle.D.Moffett@boeing.com>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>