Add a helper function to add string property updates to an OF changeset.
This is similar to of_changeset_add_prop_string(), but instead of adding
the property (and failing if it exists), it will update the property.
This shall be used later in the DT hardware prober.
Signed-off-by: Chen-Yu Tsai <wenst@xxxxxxxxxxxx>
Reviewed-by: Rob Herring (Arm) <robh@xxxxxxxxxx>