AnandTech speculates on a 128-bit DRAM bus[1], but AFAIK Apple hasn't revealed rich details about the memory architecture. It'll be interesting to see what the overall memory bandwidth story looks like as hardware trickles out.
Apple being Apple, we won't know much before someone grinds down a couple chips to reveal what the interconnections are, but if you are feeding GPUs along with CPUs, a wider memory bus between DRAM and the SoC cache makes a lot of sense.
[1] https://www.anandtech.com/show/16226/apple-silicon-m1-a14-de...