Okay, thanks already for the replies. 
Do I understand this correctly:
embedded-hal is meant to provide simplified access to hardware / interfaces when programming for embedded devices. 
The raspberry pi is technically no real embedded / edge device, but instead is considered a full linux machine (in a standard installation). 
The library I linked is built upon embedded-hal so it can be used in any context where the embedded-hal traits are implemented. 
Since RPPAL already provides a feature that implements the embedded-hal traits, I could use RPPAL to initialize an SPI device and then pass it to the initialization logic of the MFRC522 library. Am I understanding this correctly?