libfdt: Abolish fdt_offset_ptr_typed()

[Jon Loeliger]

So, like, the other day David Gibson mumbled:
> The fdt_offset_ptr_typed() macro seemed like a good idea at the time.
> However, it's not actually used all that often, it can silently throw
> away const qualifications and it uses a gcc extension (typeof) which
> I'd prefer to avoid for portability.
> 
> Therefore, this patch gets rid of it (and the fdt_offset_ptr_typed_w()
> variant which was never used at all).  It also makes a few variables
> const in testcases, which always should have been const, but weren't
> caught before because of the aforementioned silent discards.
> 
> Signed-off-by: David Gibson <david at gibson.dropbear.id.au>

Applied.

jdl