[POWERPC] Add a warning to help trackdown device_node refcounting bugs

When the refcount for a device node goes to 0, we call the
destructor - of_node_release(). This should only happen if we've
already detached the node from the device tree.

So add a flag OF_DETACHED which tracks detached-ness, and if we
find ourselves in of_node_release() without it set, issue a
warning and don't free the device_node. To avoid warning
continuously reinitialise the kref to a sane value.

Signed-off-by: Michael Ellerman <michael@ellerman.id.au>
Signed-off-by: Paul Mackerras <paulus@samba.org>