i2c: core: added documentation to the I2C core.

The I2C core bus functions have been documented for inclusion in the
Doxygen manual website.

Signed-off-by: Michel Megens <dev@michelmegens.net>
i2c: documented the client side of the I2C driver.

drivers/i2c/client.c:
	Contains the client side code of the I2C core driver.

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