platform/chrome: cros_ec_debugfs: Add debugfs entry to retrieve EC uptime

The new debugfs entry 'uptime' is being made available to userspace so that
a userspace daemon can synchronize EC logs with host time.

Signed-off-by: Tim Wawrzynczak <twawrzynczak@chromium.org>
[rework based on Tim's first approach]
Signed-off-by: Enric Balletbo i Serra <enric.balletbo@collabora.com>
Tested-by: Tim Wawrzynczak <twawrzynczak@chromium.org>