dm: i2c: Add a uclass for I2C

The uclass implements the same operations as the current I2C framework but
makes some changes to make it fit driver model better:

- Remove the chip address from API calls
- Remove the address length from API calls
- Remove concept of 'current' I2C bus
- Drop all existing init functions

Acked-by: Heiko Schocher <hs@denx.de>
Reviewed-by: Masahiro Yamada <yamada.m@jp.panasonic.com>
Signed-off-by: Simon Glass <sjg@chromium.org>