Implement MFRC522 driver, highly integrated RFID reader/writer

[forGGe]

The MFRC522 is a highly integrated reader/writer IC for contactless communication at 13.56 MHz. It can be found in a cheap RFID reader/writer modules like this.

UART is preferable as communication mode between MCU and MFRC522.
Unit tests required.

Additional implementation inside existing code is also required:

• stm32f4xx: configurable usart module #34 Configurable USART modes are required to support different baud rates.
• stm32f4xx: EXTI driver #35 EXTI driver will supply the Core with interrupts from MFRC522
• the bus driver: support xfer() while in IRQ  #36 FIFO read/write can be triggered while servicing EXTI IRQ. This will decrease response latency significantly.

Refs:
Datasheet.