A few days back there was the main ARM pull request for Linux 4.21 with new SoC and board support. A second ARM pull request has been submitted now during the final moments of the 4.21 kernel merge window and it offers up some new features, most notably the initial i.MX8 SoC support.
Developers had been aiming the i.MX8 support for Linux 4.21 after it wasn't ready the past few cycles contrary to some old plans to get it in ~4.18. But now with the pull request sent in on Friday by Olof Johansson, the support is now in place.
This includes the initial NXP i.MX8 support for the quad-core MX8MQ with four Cortex-A53 cores plus Cortex-M4. This also includes support for a i.MX8M EVK reference/developer board.