dts: Treat a missing 'reg property warning' as an error

As far as testing can tell, all occurences of the "Node has a unit
name, but no reg property" warnings have been fixed. To prevent them
from re-appearing we turn the warning into an error.

This is a breaking change for out-of-tree boards that are affected by
the warning.

Signed-off-by: Sebastian Bøe <sebastian.boe@nordicsemi.no>