clockdomains: connect clockdomain code to powerdomain code

Thie patch adds code to the powerdomain layer to track the
clockdomains associated with each powerdomain.

It also modifies the clockdomain code to register clockdomains
with their corresponding powerdomain when the clockdomain is registered.

Signed-off-by: Paul Walmsley <paul@pwsan.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>