platform/chrome: use macros for passthru indexes

Move passthru indexes for EC and PD devices to common header.  Also use
them instead of literal constants.

Reviewed-by: Guenter Roeck <groeck@chromium.org>
Signed-off-by: Tzung-Bi Shih <tzungbi@kernel.org>
Link: https://lore.kernel.org/r/20220609084957.3684698-4-tzungbi@kernel.org