Add config for 2 ADXL345 via RasPi pico

[muffix]

This adds a configuration supporting two ADXL345 sensors at the same time though a Raspberry Pi pico.

Resonance measurements for both the head and the bed can be done without having to change the sensors.

The two sensors should use the spi0a (GPIO 0-3) and spi1a (GPIO 9-12) buses, respectively, as described in this post: https://klipper.discourse.group/t/raspberry-pi-pico-adxl345-portable-resonance-measurement/1757/9

The configuration uses an axes mapping for the sensors and mounts here: https://www.printables.com/model/385334-sovol-sv06-adxl345-mount-printhead-and-bed

[bassamanator]

I'll see if I can come up with a slightly improved wiring diagram before I merge.

Please keep this open. Thanks

[bassamanator]

Will add diagram later. Thanks @muffix!