dt: Add empty of_match_node() macro

Add an empty macro for of_match_node() that will save
some '#ifdef CONFIG_OF' for non-dt builds.

I have chosen to use a macro instead of a function to
be able to avoid defining the first parameter.
In fact, this "struct of_device_id *" first parameter
is usualy not defined as well on non-dt builds.

Signed-off-by: Nicolas Ferre <nicolas.ferre@atmel.com>
Acked-by: Grant Likely <grant.likely@secretlab.ca>