MFC r330019: ofw_fdt: Simplify parts with new libfdt methods

libfdt now provides methods to iterate through subnodes and properties in a
convenient fashion.

Replace our ofw_fdt_{peer,child} searches with calls to their corresponding
libfdt methods. Rework ofw_fdt_nextprop to use the
fdt_for_each_property_offset macro, making it even more obvious what it's
doing.

No functional change intended.