powerpc/xics: Rename the map handler in a check handler

This moves the IRQ initialization done under the different ICS backends
in the common part of XICS. The 'map' handler becomes a simple 'check'
on the HW IRQ at the FW level.

As we don't need an ICS anymore in xics_migrate_irqs_away(), the XICS
domain does not set a chip data for the IRQ.

Signed-off-by: Cédric Le Goater <clg@kaod.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20210701132750.1475580-18-clg@kaod.org