fsl/mpic: Create and document the "single-cpu-affinity" device-tree flag

The Freescale MPIC (and perhaps others in the future) is incapable of
routing non-IPI interrupts to more than once CPU at a time.  Currently
all of the Freescale boards msut pass the MPIC_SINGLE_DEST_CPU flag to
mpic_alloc(), but that information should really be present in the
device-tree.

Older board code can't rely on the device-tree having the property set,
but newer platforms won't need it manually specified in the code.

[BenH: Remove unrelated changes, folded in a different patch]

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