I provide some details here.
1. Does 'No_stuff' mean 'open' in the circuit??
Ans: No_stuff means 'Don't mount or assemble in the PCB". The corresponding component will not be mounted on the PCB. Normally in the reference PCB designs, multiple option will be provided to check all possible configurations.
For example an switching regulator can work in two different configurations where the different configuration will be controlled by an external pull-up (high) and pull-down (low) at its input pin. For normal operation either pull-up or pull-down will be stuffed on the board. Other one will be No_stuff.
2. How about design with PMIC(DA6001)
It integrates the DCDC convertors, SMC, Clock(CK610) and MVP-6.
Is it right?? and do you recommend it?
Ans: As per my understanding, DA6001 is still not in production. When I checkd with Dialog semiconductor last October, only engineering samples available.
This is good integrated solution for Atom. But I am not sure that, whether DA6001 was completely validated with Atom. Please check with Dialog semiconductor that, whether they have any Atom evaluation board with DA6001.
3. The device what i want to design is using DC voltage (Not battery)
So I think that the required power state is only working state(S0).
I wonder it is possible to operate on working state during 2~3 hours.
Ans: As long as you supply DC input to your hardware, you should be able to operate in S0 without any time restriction.
If you need power management, with BIOS you can enable all sleep modes supported by the Atom.