dtc: Migrate "string property" checks to new framework

[Jon Loeliger]

So, like, the other day David Gibson mumbled:
> This patch converts to the new tree checking framework those checks
> which verify that certain properties (device_type, model) have a
> string value, when present.
> 
> Signed-off-by: David Gibson <david at gibson.dropbear.id.au>

Applied.

jdl