How did you check it? After some UART IO /cat/proc/interrupts indicating zero interrupt events for fsl-lpuart and non zero events for eDMA? That’s quite easy, but perhaps there are better ways.
Device tree should provide right settings, kernel config should have DMA driver enabled. BSP 2.8.7 seems having UART DMA’s enabled. I don’t see in logs on git.toradex.com, when VF DMA could be enabled or disabled. Some very old BSP IIRC had it disabled, but I think not BSP 2.8.x.