of: protect linux/of.h with CONFIG_OF

For platforms that have CONFIG_OF optional, we need to make the contents
of linux/of.h conditional on CONFIG_OF.

Signed-off-by: Jeremy Kerr <jeremy.kerr@canonical.com>
Signed-off-by: Grant Likely <grant.likely@secretlab.ca>
Acked-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Acked-by: Michal Simek <monstr@monstr.eu>