libfdt: Implement property iteration functions

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@gibson.dropbear.id.au>

This was extracted from the DTC commit:
73dca9ae0b9abe6924ba640164ecce9f8df69c5a Mon Sep 17 00:00:00 2001

Signed-off-by: Gerald Van Baren <vanbaren@cideas.com>