Implementing SPI library

[soburi]

We need to implement the SPI library to keep it compatible with standard AVR Arduino.

[DhruvaG2000]

Just documenting further details here,

1. SPI.h arduino API
2. Zephyr SPI API

[DhruvaG2000]

SPI should be included when writing a device overlay, but I'm not in a position to test it.

@soburi I could probably test the SPI implementation if you're able to raise a PR for SPI using ADXL355 sensor or something that I may have access to.

https://github.com/plasmapper/adxl355-arduino
This should be a good library to validate SPI works. Let me know if you are interested, I can definitely help but will be difficult for me to implement fully at the moment.