platform/chrome: Add cros_ec_lpc driver for x86 devices

Chromebooks have an Embedded Controller (EC) that is used to
implement various functions such as keyboard, power and battery.

The AP can communicate with the EC through different bus types
such as I2C, SPI or LPC.

The cros_ec mfd driver is then composed of a core driver that
register the sub-devices as mfd cells and provide a high level
communication interface that is used by the rest of the kernel
and bus specific interfaces modules.

Each connection method then has its own driver, which register
with the EC driver interface-agnostic interface.

Currently, there are drivers to communicate with the EC over
I2C and SPI and this driver adds support for LPC.

Signed-off-by: Bill Richardson <wfrichar@chromium.org>
Signed-off-by: Javier Martinez Canillas <javier.martinez@collabora.co.uk>
Tested-by: Gwendal Grignou <gwendal@chromium.org>
Reviewed-by: Gwendal Grignou <gwendal@chromium.org>
Signed-off-by: Olof Johansson <olof@lixom.net>