libfdt: Implement property iteration functions

[Jon Loeliger]

> For ages, we've been talking about adding functions to libfdt to allow
> iteration through properties.  So, finally, here are some.
> 
> I got bogged down on this for a long time because I didn't want to
> expose offsets directly to properties to the callers.  But without
> that, attempting to make reasonable iteration functions just became
> horrible.  So eventually, I settled on an interface which does now
> expose property offsets.  fdt_first_property_offset() and
> fdt_next_property_offset() are used to step through the offsets of the
> properties starting from a particularly node offset.  The details of
> the property at each offset can then be retrieved with either
> fdt_get_property_by_offset() or fdt_getprop_by_offset() which have
> interfaces similar to fdt_get_property() and fdt_getprop()
> respectively.
> 
> No explicit testcases are included, but we do use the new functions to
> reimplement the existing fdt_get_property() function.
> 
> Signed-off-by: David Gibson <david at gibson.dropbear.id.au>

Applied.

jdl