Allow application to define its own DTS bindings

[pdunaj]

Since DTS is not dynamic but generates set of defines all platform device types must be first defined in bindings. This prevents application from using DTS overlay to create application specific platform devices.

e.g. We would like to use GPIO pins to create abstract device "joystic". We could pass information which pins to use using config but we prefer DTS, assuming that we will create overlay for default board and define joystic there with 4 ints as its members... Problem is that to do that we need to create new yaml file that will be stored in dts/bindings.

The above story is only an example - we will have plenty of other device types that will be specific to our application.

It would be great if application could inform zephyr to use binding files defined and stored in application directory.

[pdunaj]

@SebastianBoe @pizi-nordic @carlescufi @kapi-no

[pdunaj]

it seems this one can be done by #14521