The default I2C device configuration allows to define up to two I2C devices `I2C_DEV(0)` and `I2C_DEV(1)`. `I2C_DEV(0)` is always defined if the I2C peripheral is enabled by the module `periph_spi`. The second I2C device `I2C_DEV(1)` is only defined if `I2C_DEV_1_USED` is defined by the board. This allows to use the default configuration with one or two I2C devices depending on whether other peripherals are enabled that would collide with the I2C devices.
16782: drivers/mfrc522: add new driver r=benpicco a=HendrikVE
### Contribution description
This PR adds support for the MFRC522. It is quite common in the Arduino world and it is quite cheap. The driver connects to the MFRC522 via SPI and is heavily based on the Arduino driver available [here](https://github.com/miguelbalboa/rfid). Basically it was ported, but with several improvements in readability and documentation.
### Testing procedure
The given (manual) test provides single commands for some driver functions.
19201: cpu/gd32v: add periph_i2c support r=benpicco a=gschorcht
### Contribution description
This PR provides the `periph_i2c` support and is one of a bunch of PRs that complete the peripheral drivers for GD32VF103.
The driver is a modified version of the driver for STM32F1 with some changes that were necessary to get it working on GD32V. As for STM32F1, the driver is using polling instead of interrupts for now. It will be implemented interrupt-driven later.
### Testing procedure
`tests/periph_i2c` as well as a test with any I2C sensor should work. The driver was tested with `tests/driver_l3gxxxx` and `tests/driver_bmp180`.
### Issues/PRs references
Co-authored-by: Hendrik van Essen <hendrik.ve@fu-berlin.de>
Co-authored-by: Gunar Schorcht <gunar@schorcht.net>
The default timer configuration has been changed so that `TIMER0` and `TIMER1` are always timer devices. `TIMER2` can only be used as timer device if it is not used for PWM devices. `TIMER3` and `TIMER4` are only available as timer devices if they are supported by the CPU model and not used for PWM devices.
