of: address: Don't fail a lookup just because a node has no reg property

Sometimes it doesn't make any sense for a node to have an address.
In this case device lookup will always be unsuccessful because we
currently assume every node will have a reg property. This patch
changes the semantics so that the resource address and the lookup
address will only be compared if one exists.

Things like AUXDATA() rely on of_dev_lookup to return the lookup
entry of a particular device in order to do things like apply
platform_data to a device. However, this is currently broken for
nodes which do not have a reg property, meaning that platform_data
can not be passed in those cases.

Acked-by: Rob Herring <rob.herring@calxeda.com>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Lee Jones <lee.jones@linaro.org>