dtc: Add ability to delete nodes and properties

dtc currently allows the contents of properties to be changed, and the
contents of nodes to be added to. There are situations where removing
properties or nodes may be useful. This change implements the following
syntax to do that:

    / {
        /delete-property/ propname;
        /delete-node/ nodename;
    };

or:

    /delete-node/ &noderef;

Signed-off-by: Stephen Warren <swarren@nvidia.com>
Acked-by: David Gibson <david@gibson.dropbear.id.au>