of: device_node kobject lifecycle fixes

After the move to having device nodes be proper kobjects the lifecycle
of the node needs to be controlled better.

At first convert of_add_node() in the unflattened functions to
of_init_node() which initializes the kobject so that of_node_get/put
work correctly even before of_init is called.

Afterwards introduce of_node_is_initialized & of_node_is_attached that
query the underlying kobject about the state (attached means kobj
is visible in sysfs)

Using that make sure the lifecycle of the tree is correct at all
times.

Signed-off-by: Pantelis Antoniou <panto@antoniou-consulting.com>
[grant.likely: moved of_node_init() calls, fixed up locking, and
               dropped __of_populate() hunks]
Signed-off-by: Grant Likely <grant.likely@linaro.org>