Feat allow spi clock speed change

[askuric]

At the moment we usually run the SPI sensors at 1MHz which is in many cases a lot under sensor capabilities.

Till now, we did allow changing it through the sensor.clock_speed variable or through the MagneticSensorSPIConfig_s.

But for the sake of simplicity this PR adds the posibility too.
It's backwards compatible, no changes in the API - the parameter is optional.

Example usage - running AS5048 at 5mHz

MagneticSensorSPI sensor = MagneticSensorSPI(AS5048_SPI, D4, 5000000);

Example usage - running AS5047 at 5mHz

MagneticSensorSPI sensor = MagneticSensorSPI(D4, 14, 0x3FFF, 5000000); // using default AS5047 config