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.
Reviewed by: nwhitehorn
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D14225