of: Drop ->next pointer from struct device_node

The ->next pointer in struct device_node is a hanger-on from when it was
used to iterate over the whole tree by a particular device_type property
value. Those days are long over, but the fdt unflattening code still
uses it to put nodes in the unflattened tree into the same order as node
in the flat tree. By reworking the unflattening code to reverse the list
after unflattening all the children of a node, the pointer can be
dropped which gives a small amount of memory savings.

Signed-off-by: Grant Likely <grant.likely@linaro.org>
Acked-by: Frank Rowand <frank.rowand@sonymobile.com>
Cc: Gaurav Minocha <gaurav.minocha.os@gmail.com>