ARM: OMAP2+: Powerdomain: Remove the need to always have a voltdm associated to a pwrdm

The powerdomain framework currently expects all powerdomains to be associated with
a corresponding voltagedomain. For some SoCs' (like the already existing AM33xx
family, or for the upcoming AM437x and DRA7 SoCs') which
do not have a Voltage controller/Voltage Processor (neither the SR I2C
bus to communicate with the PMIC) there is no need for a Powerdomain to have
a voltage domain association since there is no auto-scaling of voltages possible
using the voltage FSM.

Extend the arch operations to add an api which the powerdomain core can
then use to identify if a voltdm lookup and association for a powerdomain
is really needed.

Acked-by: Nishanth Menon <nm@ti.com>
Acked-by: Vaibhav Hiremath <hvaibhav@ti.com>
Signed-off-by: Rajendra Nayak <rnayak@ti.com>
Acked-by: Santosh Shilimkar <santosh.shilimkar@ti.com>
Tested-by: Afzal Mohammed <afzal@ti.com> # am335x evm
Signed-off-by: Kevin Hilman <khilman@linaro.org>