Add SPI clock divisor for compatibility with legacy SPI library for AVR

[Rotzbua]

Problem

Compatibility problems with libraries who use hard coded old SPI constants.

Example

MFRC522.h:22:26: error: 'SPI_CLOCK_DIV4' was not declared in this scope
miguelbalboa/rfid#544

Possible solution

Add constants as done in samd implementation:
arduino/ArduinoCore-samd@98341bf

[facchinm]

Hi @Rotzbua ,
since SPI_CLOCK_DIV4 is directly used here as frequency parameter and not as .setClockDivider() (https://github.com/arduino/ArduinoCore-avr/blob/master/libraries/SPI/src/SPI.h#L302) I think the library should be patched to conform to SPISettings specifications (https://www.arduino.cc/en/Reference/SPISettings) more than patching this core to accept bogus values 🙂
Also, samd will probably try to configure the SPI at 12Hz, not sure what happens, but it's definitely a bug in the library.

[Rotzbua]

@facchinm Thanks for the hint. So basically the following code is BS?

SPI.beginTransaction(SPISettings(SPI_CLOCK_DIV4, MSBFIRST, SPI_MODE0));

Because SPISettings needs as argument the clock speed as Hz but SPI_CLOCK_DIV4 does not contain the clock speed?