platform/chrome: cros-ec: Record event timestamp in the hard irq

To improve sensor timestamp precision, given EC and AP are in different
time domains, the AP needs to try to record the exact moment an event
was signalled to the AP by the EC as soon as possible after it happens.

First thing in the hard irq is the best place for this.

Signed-off-by: Gwendal Grignou <gwendal@chromium.org>
Acked-by: Jonathan Cameron <Jonathan.Cameron@kernel.org>
Acked-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Enric Balletbo i Serra <enric.balletbo@collabora.com>