device: Add dev_of_node() accessor

Suggested by Arnd Bergmann, this gives a practical accessor for
the of_node field of struct device while instructing the compiler
that it will be NULL if CONFIG_OF is not set.

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>