powerpc/mpic: Assume a device-node was passed in mpic_alloc()

All of the existing callers of mpic_alloc() pass in a non-NULL
device-node pointer, so the checks for a NULL device-node may be
removed.

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