Hello!
I have never played with the Sitara boards, but an SPI screen is really easy to use.
If you find a way to use GPIO (try to blink a LED for example) and a SPI, then you just
wire the SPI to the LCD, the GPIOs to the proper places (usually chip select, Data/Command,
Reset) and you're on the right way.
Usually you can find the initialization procedure anywhere on the ent.
I'm sure if you google SSD1309.c, you will find some code. I just checked, there are sources.
Dora.