libfdt: Add more documentation (path the fifth)

[Jon Loeliger]

So, like, the other day David Gibson mumbled:
> This patch documents a few more functions in libfdt.h.  All the
> read-only functions are now documented.
> 
> Signed-off-by: David Gibson <david at gibson.dropbear.id.au>

Applied.

Thanks,
jdl