x86: i2c: Add a stub driver for Intel I2C/SMbus

This is used on most Intel platforms. We don't have a driver for it yet, but
add a stub to handle the init. For now this targets ivybridge so we may want
to add a device tree binding and generalise it when other platforms are
supported.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Heiko Schocher <hs@denx.de>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>