platform/chrome: Add a new interrupt path for cros_ec_lpc

This commit allows cros_ec_lpc to register a direct IRQ instead of relying
on the ACPI notification chain to receive MKBP events.

This change is done in the interest of allowing reduced jitter in the
communication path between the CrOS EC and the host for receiving sensor
data.

Signed-off-by: Enrico Granata <egranata@chromium.org>
Signed-off-by: Benson Leung <bleung@chromium.org>