LS1021ATSN board is the early product, it includes a regular LS1021A SoC and an external L2 switch (SJA1105TEL) with TSN features. LS1021A processor is just used to configure SJA1105TEL.
Probably You could consider new TSN silicon LS1028A, it consists a new TSN-enabled Ethernet controller with endpoint-specific features, as well as a new on-chip TSN switch.
For LS1021ATSN, you could use NXP released Real-time Edge Software.
For LS1028ARDB, you could use NXP released LSDK or Real-time Edge Software.
Both LSDK and Real-time Edge Software are free for customers.
Flex Ethernet and switch drivers are related with LS1028A processor.