Merge #129

129: PLIC peripheral r=romancardenas a=romancardenas

Same as #125 (closing right now) but with a clean commit history. I've been playing around with this implementation and detected nice-to-have features and boring patterns that could be implemented using macros. Pointers to repos that are already working with it:

- [`e310x` PAC](https://github.com/greenlsi/e310x)
- [`e310x-hal` HAL](https://github.com/greenlsi/e310x-hal)
- [`hifive1` BSP](https://github.com/romancardenas/hifive1) with an illustrative [example](https://github.com/romancardenas/hifive1/blob/master/examples/interrupt.rs

)

Co-authored-by: Román Cárdenas <rcardenas.rod@gmail.com>