Add "legacy" type of irqdomain to preserve old-style numbering
and allow smooth transition for both DT and non-DT cases.

Original idea and code by Jean-Christophe Plagniol-Villard.
Signed-off-by: Nicolas Ferre <nicolas.ferre@atmel.com>
Acked-by: Jean-Christophe PLAGNIOL-VILLARD <plagnioj@jcrosoft.com>