of: Add todo tasklist for Devicetree

There are a number of outstanding workitems that need to be done on the
device tree code. Start tracking them so that interested developers can
pick up a task and start working on it.
Signed-off-by: NGrant Likely <grant.likely@linaro.org>

Documentation/devicetree/todo.txt

+Todo list for devicetree:
+
+=== General structure ===
+- Switch from custom lists to (h)list_head for nodes and properties structure
+- Remove of_allnodes list and iterate using list of child nodes alone
+
+=== CONFIG_OF_DYNAMIC ===
+- Switch to RCU for tree updates and get rid of global spinlock
+- Document node lifecycle for CONFIG_OF_DYNAMIC
+- Always set ->full_name at of_attach_node() time
+- pseries: Get rid of open-coded tree modification from arch/powerpc/platforms/pseries/dlpar.c