of: Add stub for of_add_property()

If building with OF Kconfig disabled, this can lead to errors for
drivers utilizing of_add_property().  Add a stub for the add API, as
it exists for the remove variant as well, and to avoid compliation
issues.  Also, export this API so that it can be used by modules.

Signed-off-by: Wesley Cheng <wcheng@codeaurora.org>
Link: https://lore.kernel.org/r/1625908395-5498-5-git-send-email-wcheng@codeaurora.org


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>