[dtc] Remove check for linux,platform property in /chosen

[Jon Loeliger]

So, like, the other day David Gibson mumbled:
> Jon, please apply:
> 
> The linux,platform property in /chosen was obsolete almost as soon as
> it was invented.  Remove the check for it from dtc, which just tends
> to lead to irritating spurious failures.  
> 
> Signed-off-by: David Gibson <dwg at au1.ibm.com>

Done.

Thanks,
jdl