mfd: cros_ec: Add support for dumping panic information

This dumps the EC panic information from the previous reboot.

Similar to the information presented by ectool panicinfo, except
that we do not bother doing any parsing (we should write a small
offline tool for that).

Signed-off-by: Nicolas Boichat <drinkcat@chromium.org>
Reviewed-by: Guenter Roeck <groeck@chromium.org>
Tested-by: Enric Balletbo i Serra <enric.balletbo@collabora.com>
Signed-off-by: Benson Leung <bleung@chromium.org>