No way to get clock control subsystem type

[askawu]

The clock control API is introduced since 1.9. clock_control_subsys_t is an opaque data structure used to abstract the platform-dependent information. However, there's no way for an application to get a clock_control_subsys_t from somewhere and use it to access clock control API.

For example, on the frdm k64f, the clock control should be configured correctly before accessing pins in port D because the clock control of port D is disabled by default.

IMHO, we should provide a function like device_get_binding() to get the desired clock control subsystem.

[carlescufi]

@askawu I see that device_get_binding() is being used to obtain a reference to an instance of the clock subsystem. Closing this, please reopen if you think this is still an issue.