of/address: Handle #address-cells > 2 specially

When a bus specifies #address-cells > 2, of_bus_default_map() now
assumes that the mapping isn't for a physical address but rather an
identifier that needs to match exactly.

This is required by bindings that use multiple cells to translate a
resource to the parent bus (device index, type, ...).

See here for the discussion:

	https://lists.ozlabs.org/pipermail/devicetree-discuss/2012-June/016577.html

Originally-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Thierry Reding <thierry.reding@avionic-design.de>
Signed-off-by: Rob Herring <rob.herring@calxeda.com>