irq_domain: Centralize definition of irq_dispose_mapping()

Several architectures define their own empty irq_dispose_mapping().  Since
the irq_domain code is centralized now, there is little need to do so.  This
patch removes them and creates a new empty copy when !CONFIG_IRQ_DOMAIN is
selected.

The patch also means that IRQ_DOMAIN becomes selectable on all architectures.

Signed-off-by: Grant Likely <grant.likely@secretlab.ca>
Cc: Rob Herring <rob.herring@calxeda.com>
Cc: "David S. Miller" <davem@davemloft.net>
Cc: Jonas Bonn <jonas@southpole.se>
Cc: sparclinux@vger.kernel.org
Cc: linux@lists.openrisc.net