I have spi2 working on P9.18,21 via /dev/spidev1.0 same as the BBB.
However there is an important difference. The default assignment of spi master mode mosi/miso to P9.18,21 are reversed compared to the BBB unless you add
to the mcspi2 section of your .dts file. Details are in this Github issue: https://github.com/beagleboard/beaglebone-ai/issues/28