A Cortex-M0+ core - which is a really tiny microcontroller - can do TLS 1.2 just fine (using the AES-CCM AEAD instead of the AES-GCM AEAD helps somewhat, apparently: I've not tried to implement it myself, so I'm not clear precisely why, but it's probably GHASH). With enough work, an 8-bit class chip with a couple kilobytes of RAM could implement a constrained subset. If that's somehow still too heavy, I'm not sure how.

I hope they can find a way to make TLS 1.3 work for their IoT scenarios: CHACHA20_POLY1305 and Curve25519 will also hopefully help, quite a lot. They're as small as they are fast.