powerpc/xics: Set the IRQ chip data for the ICS native backend

The ICS native driver relies on the IRQ chip data to find the struct
'ics_native' describing the ICS controller but it was removed by commit
248af248 ("powerpc/xics: Rename the map handler in a check handler").
Revert this change to fix the Microwatt SoC platform.

Fixes: 248af248 ("powerpc/xics: Rename the map handler in a check handler")
Signed-off-by: Cédric Le Goater <clg@kaod.org>
Tested-by: Gustavo Romero <gustavo.romero@linaro.org>
Reviewed-by: Joel Stanley <joel@jms.id.au>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20210913134056.3761960-1-clg@kaod.org