powerpc/xics: Add support for IRQ domain hierarchy

XICS doesn't have any state associated with the IRQ. The support is
straightforward and simpler than for XIVE.

Signed-off-by: Cédric Le Goater <clg@kaod.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20210701132750.1475580-21-clg@kaod.org