At present, it seems that the current Linux kernel that's supported and maintained for the i.MX7D has provision for only two operating-points ("ARM frequency - Voltage" pairs) usable by the Linux power-management framework. This can be seen in the code too: ( linux-2.6-imx.git - Freescale i.MX Linux Tree)
That is, basically, frequencies 1GHz and 800MHz are supported right now. Can the i.MX7D support operating-points with frequencies less than 800MHz like 600MHz, 400MHz and 200MHz? If yes, then is some work already underway for implementing the same? Any idea on when that code will be GA'ed?