I2C driver implementation (Master mode) for the SAMR21

Implementation of the I2C driver (Master mode) for the SAMR21 including:

- periph_conf.h: I2C configuration added
- i2c.c: I2C_0, implementation of all functions but slave mode

I do some "return;" when I detect an error which is not that good. Which solution do I have?
Please use existing defines (`instance_sercom3.h`)
```C
#define SERCOM3_GCLK_ID_CORE        23
#define SERCOM3_GCLK_ID_SLOW        19
```
BTW: Coding conventions reserve CAPITAL_NAMES to defines.