UPSTREAM: of: Use vargs in __of_node_alloc

The overlay code needs to construct a new full_name from the parent name
and the node name, but the current method has to allocate and then free
an temporary string which is wasteful. Fix this problem by using vargs
to pass in a format and arguments into __of_node_alloc().

At the same time remove the allocflags argument to __of_node_alloc().
The only users all use GFP_KERNEL, so there is no need to provide it as
an option. If there is ever a need later it can be added back.

Signed-off-by: Grant Likely <grant.likely@linaro.org>
(cherry picked from commit ef8bbd73a76197cf8362a2b43aaadc5717bd0746)
Bug: 109890997
Change-Id: Ifeb13b49c325232bebea5e4fe62fee26f8736479
Signed-off-by: John Stultz <john.stultz@linaro.org>