dm: i2c: Add I2C emulation driver for sandbox

In order to test I2C we need some sort of emulation interface. Add hooks
to allow a driver to emulate an I2C device for sandbox.

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