of/irq: Add interrupts-names property to name an irq resource

Add a interrupts-names property to allow the possibility to provide a name
to any interrupts entries.  If the name is available, use it to name the
resource, otherwise keep the device full name.

Signed-off-by: Benoit Cousson <b-cousson@ti.com>
Cc: Grant Likely <grant.likely@secretlab.ca>
Cc: Rob Herring <rob.herring@calxeda.com>
[grant.likely: use "interrupt-names" and tidy documentation]
Signed-off-by: Grant Likely <grant.likely@secretlab.ca>