of: introduce of_get_available_child_count

Some drivers keep counting available child by themselves. So
introduce a new simple API like of_get_child_count() but for
available childs.

Cc: Josh Wu <josh.wu@atmel.com>
Signed-off-by: Bryan Wu <cooloney@gmail.com>
Acked-by: Rob Herring <rob.herring@calxeda.com>