the Build Environment for LPC54608 is prepared to work with the on-chip LCD controller. In your case you want to transfer the content of the framebuffer to an external graphics memory via SPI.
The complete interaction with the LCD controller is implemented in the file ew_bsp_display.c. You can adapt this file and implement your own SPI driver to copy the content of the framebuffer to your display memory.
There is one Build Environment that contains a similar approach - it is the STM32F407 Build Environment. Maybe it is helpful to have a look into this package (ew_bsp_display.c, main.c and ewextgfx.c).
But it will be not so easy to change the complete framebuffer concept of a Build Environment - typically we offer such bring-ups of custom specific hardware as services.