You can hard reset all the chips by connecting the RSTN lines together and pulling them to GND.
You can do this either with the MCU or a button (or both)
There is no way of soft resetting all the chips using one command.
Each chip has it's own Shutdown register and you have to clear the shdn bit in each chip seperately.
There might be a way of resetting them all with software but I haven't tried it and the datasheet is not clear about it but you can get all the chips to respond on the same address which may mean that one write to the Shutdown reg. will do them all. You'll have to try it to find out. 