mfd: cros_ec: add debugfs, console log file

If the EC supports the new CONSOLE_READ command type, then we
place a console_log file in debugfs for that EC device which allows
us to grab EC logs. The kernel will poll every 10 seconds for the
log and keep its own buffer, but userspace should grab this and
write it out to some logs which actually get rotated.

Signed-off-by: Eric Caruso <ejcaruso@chromium.org>
Signed-off-by: Nicolas Boichat <drinkcat@chromium.org>
Acked-by: Lee Jones <lee.jones@linaro.org>
Tested-by: Enric Balletbo i Serra <enric.balletbo@collabora.com>
[bleung: restored original version of this commit, with pointer size
 issue to be fixed in next commit]
Signed-off-by: Benson Leung <bleung@chromium.org>