mfd: cros_ec: Allow static din/dout buffers with cros_ec_register()

The lower-level driver may want to provide its own buffers. If so,
there's no need to allocate new ones.  This already happens to work
just fine (since we check for size of 0 and use devm allocation), but
it's good to document it.

[dianders: Resolved conflicts; documented that no code changes needed
on mainline]

Signed-off-by: Bill Richardson <wfrichar@chromium.org>
Signed-off-by: Doug Anderson <dianders@chromium.org>
Reviewed-by: Simon Glass <sjg@chromium.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>