dtc: Use libfdt endian conversion functions in libfdt

Following on from the last patch, which made dtc use the same endian
conversion functions as libfdt, this patch makes ftdump use these
functions as well.  This brings us down to a single set of endian
handling functions in all of dtc and libfdt, so just one place to fix
things.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>