The STKa8MPxL supports HDMI. If the BSP version is Scarthgap, the HDMI functionality is included in the default device tree.
For older BSP revisions (Kirkstone and below), the correct device tree needs to be configured in U-Boot as described below:
setenv fdtfile imx8mp-tqma8mpql-mba8mpxl-hdmi.dtbsaveenv commandboot command