[POWERPC] Simplify xics direct/lpar irq_host setup

The xics code currently has a direct and lpar variant of
xics_host_map, the only difference being which irq_chip they use.  If
we remember which irq_chip we're using we can combine these two
routines.  That also allows us to have a single irq_host_ops instead
of two.

Signed-off-by: Michael Ellerman <michael@ellerman.id.au>
Acked-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Paul Mackerras <paulus@samba.org>