mfd / platform: cros_ec: Handle chained ECs as platform devices

An MFD is a device that contains several sub-devices (cells). For instance,
the ChromeOS EC fits in this description as usually contains a charger and
can have other devices with different functions like a Real-Time Clock,
an Audio codec, a Real-Time Clock, ...

If you look at the driver, though, we're doing something odd. We have
two MFD cros-ec drivers where one of them (cros-ec-core) instantiates
another MFD driver as sub-driver (cros-ec-dev), and the latest
instantiates the different sub-devices (Real-Time Clock, Audio codec,
etc).

                  MFD
------------------------------------------
   cros-ec-core
       |___ mfd-cellA (cros-ec-dev)
       |       |__ mfd-cell0
       |       |__ mfd-cell1
       |       |__ ...
       |
       |___ mfd-cellB (cros-ec-dev)
               |__ mfd-cell0
               |__ mfd-cell1
               |__ ...

The problem that was trying to solve is to describe some kind of topology for
the case where we have an EC (cros-ec) chained with another EC
(cros-pd). Apart from that this extends the bounds of what MFD was
designed to do we might be interested on have other kinds of topology that
can't be implemented in that way.

Let's prepare the code to move the cros-ec-core part from MFD to
platform/chrome as this is clearly a platform specific thing non-related
to a MFD device.

  platform/chrome  |         MFD
------------------------------------------
                   |
   cros-ec ________|___ cros-ec-dev
                   |       |__ mfd-cell0
                   |       |__ mfd-cell1
                   |       |__ ...
                   |
   cros-pd ________|___ cros-ec-dev
                   |        |__ mfd-cell0
                   |        |__ mfd-cell1
                   |        |__ ...

Signed-off-by: Enric Balletbo i Serra <enric.balletbo@collabora.com>
Acked-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Reviewed-by: Gwendal Grignou <gwendal@chromium.org>
Tested-by: Gwendal Grignou <gwendal@chromium.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>