No spidev on Rock Pi S

Installing and Using OpenWrt
1

I have just installed 24.10 on a Radxa Rock Pi S with a view to using it to make a weighing scale with a HX711 chip.

The HX711 communicates through SPI (kind of) but out of the box there is no /dev/spidev*. I have installed kmod-spi-dev but when I modprobe spidev it does not show up with lsmod. insmod says "Failed to find spidev. Maybe it is a built in module ?"

So how do I get /dev/spidev to appear?

gpioinfo says (in part):

gpiochip1 - 32 lines:
        line  22:       "header1-pin21 [GPIO1_C6]"      input
        line  23:       "header1-pin19 [GPIO1_C7]"      input
        line  24:       "header1-pin23 [GPIO1_D0]"      input
        line  25:       "header1-pin24 [GPIO1_D1]"      input

Those are the pins I want to use for SPI2.