of.h: Keep extern declaration of of_* variables when !CONFIG_OF

Keep the extern declaration of of_allnodes and friends, when building without
of support, this way code using them can be written like this:

	if (IS_ENABLED(CONFIG_OF_PLATFORM) && of_chosen) {
		for_each_child_of_node(of_chosen, np)
			...
	}

And rely on the compiler optimizing it away, avoiding the need for #ifdef-ery.

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>