powerpc/mpic: Fix mapping of "DCR" based MPIC variants

Commit 31207dab


"Fix incorrect allocation of interrupt rev-map"
introduced a regression crashing on boot on machines using
a "DCR" based MPIC, such as the Cell blades.

The reason is that the irq host data structure is initialized
much later as a result of that patch, causing our calls to
mpic_map() do be done before we have a host setup.

Unfortunately, this breaks _mpic_map_dcr() which uses the
mpic->irqhost to get to the device node.

This fixes it by, instead, passing the device node explicitely
to mpic_map().

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Acked-by: Akira Tsukamoto <akirat@rd.scei.sony.co.jp>