libfdt: Add and use a node iteration helper function.

[Jon Loeliger]

So, like, the other day David Gibson mumbled:
> 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 at gibson.dropbear.id.au>
> 
> ---
> 
> I think we're ready to go with this one.  I'm still thinking about
> suitable for_each_* macros, but in the mean time I'm happy with the
> exported interface here, and it's a code-reducing patch.

Applied.

Thanks,
jdl