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.

Change-Id: If37ca9fd66061d2feceda37b0f2e66c9e0ae8683
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>
[mbohan@codeaurora.org: resolve conflict in drivers/of/irq.c]
Signed-off-by: Michael Bohan <mbohan@codeaurora.org>