Hi,
For the LPC55S06, as you know that the SPI requires 4 signals, the uart requires only 2 signals. Because of pin limited, the FC2 and FC5 does not support SPI, but they also support uart, I2C and I2S functions.
It is wrong to define:
#define SPI_IRQS { FLEXCOMM0_IRQn, FLEXCOMM1_IRQn, FLEXCOMM3_IRQn, FLEXCOMM4_IRQn, FLEXCOMM6_IRQn, FLEXCOMM7_IRQn, FLEXCOMM8_IRQn }
Because the FC2 and FC5 can not be configured as spi for LPC55S06

Hope it can help you
BR
XiangJun Rong