I am afraid that we do not have any examples for an SPI bootloader for the nRF52. You will have to port the SPI bootloder example for the nRF51 to the nRF52.
I am afraid that we do not have any examples for an SPI bootloader for the nRF52. You will have to port the SPI bootloder example for the nRF51 to the nRF52.