of: restructure for_each macros to fix compile warnings

Commit 00b2c76a "include/linux/of.h: make for_each_child_of_node()
reference its args when CONFIG_OF=n" fixed warnings for unused
variables, but introduced variable "used uninitialized" warnings.
Simply initializing the variables would result in "set but not used"
warnings with W=1.

Fix both types of warnings by making all the for_each macros
unconditional and rely on the dummy static inline functions to
initialize and reference any variables.

Reported-by: Geert Uytterhoeven <geert@linux-m68k.org>
Cc: David Howells <dhowells@redhat.com>
Signed-off-by: Rob Herring <robh@kernel.org>
Acked-by: Grant Likely <grant.likely@linaro.org>