Dear @temalab,
How are you? As you can see in the menuconfig from the Kernel, the CONFIG_SPI_SPIDEV has a few dependencies. To use it as built-in, you should also ensure that SPI and SPI_MASTER are properly listed as built-in features.
On the Upstream Kernel of BSP 5.6.0, the .config looks like:
$ cat .config | grep CONFIG_SPI
# CONFIG_SPI_CADENCE_QUADSPI is not set
# CONFIG_SPI_MTK_QUADSPI is not set
CONFIG_SPI=y
# CONFIG_SPI_DEBUG is not set
CONFIG_SPI_MASTER=y
CONFIG_SPI_MEM=y
# CONFIG_SPI_ALTERA is not set
# CONFIG_SPI_AXI_SPI_ENGINE is not set
CONFIG_SPI_BITBANG=y
# CONFIG_SPI_CADENCE is not set
# CONFIG_SPI_DESIGNWARE is not set
# CONFIG_SPI_FSL_LPSPI is not set
CONFIG_SPI_FSL_QUADSPI=y
# CONFIG_SPI_NXP_FLEXSPI is not set
CONFIG_SPI_GPIO=y
CONFIG_SPI_IMX=y
# CONFIG_SPI_FSL_SPI is not set
# CONFIG_SPI_OC_TINY is not set
# CONFIG_SPI_PL022 is not set
# CONFIG_SPI_PXA2XX is not set
# CONFIG_SPI_ROCKCHIP is not set
# CONFIG_SPI_SC18IS602 is not set
# CONFIG_SPI_SIFIVE is not set
# CONFIG_SPI_MXIC is not set
# CONFIG_SPI_XCOMM is not set
# CONFIG_SPI_XILINX is not set
# CONFIG_SPI_ZYNQMP_GQSPI is not set
CONFIG_SPI_SPIDEV=m
# CONFIG_SPI_LOOPBACK_TEST is not set
# CONFIG_SPI_TLE62X0 is not set
# CONFIG_SPI_SLAVE is not set
Did you check on your image if the requirements were also enabled? On the downstream kernel, the configuration is as follows:
$ cat .config | grep CONFIG_SPI
# CONFIG_SPI_CADENCE_QUADSPI is not set
# CONFIG_SPI_MTK_QUADSPI is not set
CONFIG_SPI=y
# CONFIG_SPI_DEBUG is not set
CONFIG_SPI_MASTER=y
CONFIG_SPI_MEM=y
# CONFIG_SPI_ALTERA is not set
# CONFIG_SPI_AXI_SPI_ENGINE is not set
CONFIG_SPI_BITBANG=y
# CONFIG_SPI_CADENCE is not set
# CONFIG_SPI_DESIGNWARE is not set
CONFIG_SPI_FSL_LPSPI=y
CONFIG_SPI_FSL_QUADSPI=y
CONFIG_SPI_NXP_FLEXSPI=y
CONFIG_SPI_GPIO=y
CONFIG_SPI_IMX=y
CONFIG_SPI_FSL_LIB=y
CONFIG_SPI_FSL_SPI=y
# CONFIG_SPI_OC_TINY is not set
CONFIG_SPI_PL022=y
# CONFIG_SPI_PXA2XX is not set
# CONFIG_SPI_ROCKCHIP is not set
# CONFIG_SPI_SC18IS602 is not set
# CONFIG_SPI_SIFIVE is not set
# CONFIG_SPI_MXIC is not set
# CONFIG_SPI_THUNDERX is not set
# CONFIG_SPI_XCOMM is not set
# CONFIG_SPI_XILINX is not set
# CONFIG_SPI_ZYNQMP_GQSPI is not set
CONFIG_SPI_SPIDEV=y
# CONFIG_SPI_LOOPBACK_TEST is not set
# CONFIG_SPI_TLE62X0 is not set
CONFIG_SPI_SLAVE=y
CONFIG_SPI_SLAVE_TIME=y
CONFIG_SPI_SLAVE_SYSTEM_CONTROL=y
CONFIG_SPI_DYNAMIC=y
Does this help you?