libfdt: Add and use a node iteration helper function.

libfdt: Add and use a node iteration helper function.

This patch adds an fdt_next_node() function which can be used to
iterate through nodes of the tree while keeping track of depth.  This
function is used to simplify the iteration code in a lot of other
functions, and is also exported for use by library users.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>