MAINTAINERS: platform/chrome: Add Enric as a maintainer

Enric has volunteered to help me with maintaining chrome-platform
as we change the development model toward strictly upstream-first for
any chrome-platform and cros_ec driver.

Signed-off-by: Benson Leung <bleung@chromium.org>
Signed-off-by: Enric Balletbo i Serra <enric.balletbo@collabora.com>