[dtc] Fix flat_dt_property structure

[Jon Loeliger]

So, like, the other day David Gibson mumbled:
> The structure in flat_dt.h defining the layout of a proprety within
> the flat device tree is incorrect.  It has the offset to the
> property's name, then the length when in fact (according to
> booting-without-of.txt and the output of dtc) then length should come
> first, followed by the name offset.
> 
> In fact, this structure is never used so the mistake doesn't break
> anything, but it should still be fixed to avoid misleading people.
> 
> Signed-off-by: David Gibson <david at gibson.dropbear.id.au>

Applied.  Thanks.

jdl