of: wrap accesses to device_node kobject

In preparation to make kobject element in struct device_node optional,
provide and use a macro to return the kobject pointer. The only user
outside the DT core is the driver core.

Change-Id: I47023bc3b123af975adf99d9bbb8e41bdffdaeb2
Acked-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Tested-by: Nicolas Pitre <nico@linaro.org>
Reviewed-by: Frank Rowand <frowand.list@gmail.com>
Acked-by: Grant Likely <grant.likely@secretlab.ca>
Signed-off-by: Rob Herring <robh@kernel.org>
Git-Commit: 0c3c234b95fa7f1dfa19e1456a47ebafc300dd6b
Git-Repo: git://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git


Signed-off-by: Patrick Daly <pdaly@codeaurora.org>