OF: make of_property_for_each_{u32|string}() use parameters if OF is not enabled

I am getting a few
|warning: unused variable ‘p’ [-Wunused-variable]
|warning: unused variable ‘prop’ [-Wunused-variable]

in the case where CONFIG_OF is not defined and the parameters are only
used in the loop macro.

Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Grant Likely <grant.likely@linaro.org>