Hi Hiroshi,
If my understanding is correct, you want to run a 1GHz i.MX6 processor at 800MHz (core frequency), right? If so, the answer is yes, you just have to configure the clocks (see reference manual chapter 18) so the resulting frequency is the one you desire. If you are using an OS like Linux, this is done automatically by configuring some settings. I'm mainly focused on HW, but a SW person can help you with this.
Best regards.
Jorge.