i2c: atmega: added a setup for the ATmega I2C bus.

Added a bus structure and basic I/O definitions for the I2C bus. The
core bus needs some extra features before the actual bus can be
implemented. Requirements:

	* Set the bus speed
	* Time out handling
	* Retry setting/handling

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