of: Add empty for_each_available_child_of_node() macro definition

Add this empty macro definition so users can be compiled without
excluding this macro call with preprocessor directives when CONFIG_OF
is disabled.

Signed-off-by: Sylwester Nawrocki <s.nawrocki@samsung.com>
Signed-off-by: Kyungmin Park <kyungmin.park@samsung.com>
Signed-off-by: Rob Herring <rob.herring@calxeda.com>