powerpc/mpic: Cache the device-tree node in "struct mpic"

Store the node pointer in the MPIC during initialization so that all of
the later operational code can just reuse the cached pointer.

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