When a powerdomain is registered and it has an associated voltage domain,
add the powerdomain to the voltagedomain using voltdm_add_pwrdm().

Also add voltagedomain iterator helper functions to iterate over all
registered voltagedomains and all powerdomains associated with a
voltagedomain.

Modeled after a similar relationship between clockdomains and powerdomains.
Signed-off-by: Kevin Hilman <khilman@ti.com>