irq_domain/x86: Convert x86 (embedded) to use common irq_domain

This patch removes the x86-specific definition of irq_domain and replaces
it with the common implementation.

Signed-off-by: Grant Likely <grant.likely@secretlab.ca>
Acked-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Cc: Rob Herring <rob.herring@calxeda.com>
Cc: Thomas Gleixner <tglx@linutronix.de>