x86: dt: Use linear irq domain for ioapic(s)

The former conversion to irq_domain_add_legacy() did not fully work
since we miss the irq decs for NR_IRQS_LEGACY+.

Ideally we could use irq_domain_add_simple() or the no-map variant (and
program the virq <-> line mapping directly into ioapic) but this would
require a different irq lookup in "do_IRQ()" and won't work with ACPI
without changes. So this is probably easiest for everyone.

Tested-by: Thierry Reding <thierry.reding@avionic-design.de>
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Cc: Grant Likely <grant.likely@secretlab.ca>
Link: http://lkml.kernel.org/r/20120813202304.GA3529@breakpoint.cc
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>