powerpc/xics: Give a name to the default XICS IRQ domain

and clean up the error path.

Signed-off-by: Cédric Le Goater <clg@kaod.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20210701132750.1475580-19-clg@kaod.org