dtlib is meant to be general and anything-goes re. property values, but
edtlib can be pickier. Check that all 'status' properties have one of
the values listed in the devicetree specification
(https://www.devicetree.org/specifications/), and error out otherwise.
This will get rid of the 'status = "ok"'s (should be "okay") that keep
cropping up.
Signed-off-by: Ulf Magnusson <Ulf.Magnusson@nordicsemi.no>
