devicetree.h: DT_FOREACH_NODE_VARGS, DT_FOREACH_STATUS_OKAY_NODE_VARGS

Add the _VARGS variant of DT_FOREACH_NODE and
DT_FOREACH_STATUS_OKAY_NODE for when we want to do some kind of
operation on all the nodes in the tree.

Signed-off-by: Carlo Caione <ccaione@baylibre.com>