mfd: cros_ec: Add MKBP event support

Newer revisions of the ChromeOS EC add more events besides the keyboard
ones. So handle interrupts in the MFD driver and let consumers register
for notifications for the events they might care.

To keep backward compatibility, if the EC doesn't support MKBP event, we
fall back to the old MKBP key matrix host command.

Cc: Randall Spangler <rspangler@chromium.org>
Cc: Vincent Palatin <vpalatin@chromium.org>
Cc: Benson Leung <bleung@chromium.org>
Signed-off-by: Vic Yang <victoryang@google.com>
Signed-off-by: Tomeu Vizoso <tomeu.vizoso@collabora.com>
Tested-by: Enric Balletbo i Serra <enric.balletbo@collabora.com>
Acked-by: Olof Johansson <olof@lixom.net>
Signed-off-by: Lee Jones <lee.jones@linaro.org>