dtc: Add may const qualifications

[Jon Loeliger]

So, like, the other day David Gibson mumbled:
> This adds 'const' qualifiers to many variables and functions.  In
> particular it's now used for passing names to the tree accesor
> functions.
> 
> Signed-off-by: David Gibson <david at gibson.dropbear.id.au>


Typo fixed, rebased and applied.

Thanks,
jdl