Apologies. That's correct, the IMXRT1060 does not support this register. It is an typo that I will report to the corresponding department.
In such a case, you will need to create an algorithm using a general purpose timer as the time base connected to the TRIGGER and take a snapshot of the POSD register in order to calculate speed.
On the other hand, you can use IMXRT1160/70 where it is supported.
Wish it helps you.