cpu/samd21: rewrite i2c_init_master to make use of gpio_init_sercom

The former i2c_init_master was too complicated. It was trying to set IN/OUT
mode of the pins, but all that is needed is to set the proper MUX.

Also the configuration for the boards was incomplete (no MUX, no pad
setting).

It was tested on a SODAQ Autonomo, but not on a samr21-xpro