It looks like the i.MX6 SABRE-SD cannot boot from SPI NOR flash. See the highlighted known issue below, which comes from i.MX6 Linux release note.
However, the i.MX6 SABRE-AI board can boot from SPI NOR. It uses EIM_D16/17/18/19 pins.
Is there a way to use other ECSPI ports to support bootup by i.MX6 ROM bootstrap?