i2c: ChromeOS EC tunnel driver

On ARM Chromebooks we have a few devices that are accessed by both the
AP (the main "Application Processor") and the EC (the Embedded
Controller).  These are:
* The battery (sbs-battery).
* The power management unit tps65090.

On the original Samsung ARM Chromebook these devices were on an I2C
bus that was shared between the AP and the EC and arbitrated using
some extranal GPIOs (see i2c-arb-gpio-challenge).

The original arbitration scheme worked well enough but had some
downsides:
* It was nonstandard (not using standard I2C multimaster)
* It only worked if the EC-AP communication was I2C
* It was relatively hard to debug problems (hard to tell if i2c issues
  were caused by the EC, the AP, or some device on the bus).

On the HP Chromebook 11 the design was changed to:
* The AP/EC comms were still i2c, but the battery/tps65090 were no
  longer on the bus used for AP/EC communication.  The battery was
  exposed to the AP through a limited i2c tunnel and tps65090 was
  exposed to the AP through a custom Linux driver.

On the Samsung ARM Chromebook 2 the scheme is changed yet again, now:
* The AP/EC comms are now using SPI for faster speeds.
* The EC's i2c bus is exposed to the AP through a full i2c tunnel.

The upstream "tegra124-venice2" uses the same scheme as the Samsung
ARM Chromebook 2, though it has a different set of components on the
other side of the bus.

This driver supports the scheme used by the Samsung ARM Chromebook 2.
Future patches to this driver could add support for the battery tunnel
on the HP Chromebook 11 (and perhaps could even be used to access
tps65090 on the HP Chromebook 11 instead of using a special driver,
but I haven't researched that enough).

Signed-off-by: Vincent Palatin <vpalatin@chromium.org>
Signed-off-by: Simon Glass <sjg@chromium.org>
Signed-off-by: Doug Anderson <dianders@chromium.org>
Reviewed-by: Wolfram Sang <wsa@the-dreams.de>
Signed-off-by: Lee Jones <lee.jones@linaro.org>

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