of/address: Add reg-names property to name an iomem resource

Add a reg-names property to allow for reg regions to be reference by name
instead of by index.  Some devices have multiple register regions which
are more naturally referenced by name.

If the name is available, use it to name the resource when creating a devices.
Otherwise keep the device name.

Signed-off-by: Benoit Cousson <b-cousson@ti.com>
Cc: Grant Likely <grant.likely@secretlab.ca>
Cc: Rob Herring <rob.herring@calxeda.com>
[Generalized documentation to be for any -names property]
Signed-off-by: Grant Likely <grant.likely@secretlab.ca>