i2c: atmega: implemented the a speed configurator

The speed of the ATmega AVR I2C bus can be changed and configured using
atmega_i2c_setspeed. Other drivers (i2c clients) can use
bus::ctrl(bus, I2C_SET_SPEED, &speed). The speed argument is given in
Hertz. The default SCL frequency (set on initialise) is 100KHz.

Signed-off-by: Michel Megens <dev@michelmegens.net>