doc: i2c: added a module for I2C drivers.

Added two modules:
	* i2c
	* i2c-atmega

The i2c module is used for the I2C core driver and contains all modules
for platform specific I2C bus drivers, for example the i2c-atmega.

Signed-off-by: Michel Megens <dev@michelmegens.net>
i2c: added the I2C core to its doxygen module.

Added the I2C core driver to the correct I2C doxygen module: i2c. The
atmega I2C bus driver will be added to the i2c-atmega module, as a child
of the i2c module.

dev: core: Removed an old module documentation definition.

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