scripts: dts: Replace 'sub-node:' with more general 'child-binding:'

Deprecate 'sub-node:' and add a more general 'child-binding:' mechanism
to bindings. Keep supporting 'sub-node:', but print a deprecation
warning when it's used.

Like 'sub-node:', 'child-binding:' gives a binding to child nodes, but
the binding is required to be a complete binding, and is treated (and
checked) like a normal binding.

'child-binding:' can in turn contain another 'child-binding:', up to any
number of levels. This is automatic from treating it like a normal
binding, and from the code initializing parent Devices before child
Devices.

This lets nodes give bindings to grandchildren.

For example, take this devicetree fragment:

    parent {
            compatible = "foo";
            child-1 {
                    grandchild-1 {
                            ...
                    };
                    grandchild-2 {
                            ...
                    };
            };
            child-2 {
                    grandchild-3 {
                            ...
                    };
            };
    };

The binding for 'foo' could provide bindings for grandchild-1/2/3 like
this:

    compatible: "foo"

    # Binding for children
    child-binding:
        title: ...
        description: ...

        ...

        # Binding for grandchildren
        child-binding:
            title: ...
            description: ...

            properties:
                ...

Due to implementation issues with the old devicetree scripts, only two
levels of 'child-binding:' is supported for now. This limitation will go
away in Zephyr 2.2.

Piggyback shortening 'description:' and 'title:' in some bindings that
provide child bindings. This makes the generated header a bit neater.

Signed-off-by: Ulf Magnusson <Ulf.Magnusson@nordicsemi.no>