powerpc/xics: Remove ICS list

We always had only one ICS per machine. Simplify the XICS driver by
removing the ICS list.

The ICS stored in the chip data of the XICS domain becomes useless and
we don't need it anymore to migrate away IRQs from a CPU. This will be
removed in a subsequent patch.

Signed-off-by: Cédric Le Goater <clg@kaod.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20210701132750.1475580-17-clg@kaod.org