defconfig: Enable the HX711 load-cell ADC driver

[mason9468]

Enable CONFIG_HX711=m in the five Raspberry Pi kernel configurations so kernel packages include the existing Linux IIO driver for HX711 load-cell boards:

• arch/arm/configs/bcmrpi_defconfig
• arch/arm/configs/bcm2709_defconfig
• arch/arm64/configs/bcm2711_defconfig
• arch/arm64/configs/bcm2711_rt_defconfig
• arch/arm64/configs/bcm2712_defconfig

I’m using HX711 hardware with SpoolBuddy and have tested the separately built Linux driver on Raspberry Pi 3B, 4B and 5. Currently, users need to build the driver separately for their installed kernel. Including it in the kernel packages would remove that requirement and let the driver update with the kernel.

This enables the existing driver as a module. GPIO assignments still require a device-tree description.

Testing:

• The patch adds one configuration line per file and applies cleanly to rpi-6.18.y.
• The branch’s HX711 source builds against Raspberry Pi 6.18.50 headers for both v8 and 2712.
• The earlier Pi 3B, 4B and 5 hardware tests used the separately built Linux v6.18 driver, rather than a kernel built from this patch.
• The additional 32-bit and real-time configurations have not been build- or hardware-tested. I have not built or booted a complete kernel image from these changed configurations.

An equivalent patch is prepared for rpi-6.12.y if a backport is wanted. That branch’s driver also builds against 6.12.109 v8 and 2712 headers.

Related application work: maziggy/bambuddy#3187