scripts: dts: Generalize '#cells' to allow multiple sources

Implement a nice generalization suggested by Bobby Noelte.

Instead of having a generic #cells key in bindings, have source-specific
*-cells keys. Some examples:

    interrupt-cells:
        - irq
        - priority
        - flags

    gpio-cells:
        - pin
        - flags

    pwm-cells:
        - channel
        - period

This makes bindings a bit easier to read, and allows a node to be a
controller for many different 'phandle-array' properties.

The prefix before *-cells is derived from the property name, meaning
there's no fixed set of *-cells keys. This is possible because of the
earlier 'phandle-array' generalization.

The older #cells key is supported for backwards compatibility, but
generates a deprecation warning.

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