mainboards,ec: provide common declaration for mainboard_ec_init()

Add a header file to provide common declarations that the
mainboards can use regarding EC init.

BUG=chrome-os-partner:56677

Change-Id: Iaa0b37eff4de644e969a18364713b90b7f27fa1c
Signed-off-by: Aaron Durbin <adurbin@chromium.org>
Reviewed-on: https://review.coreboot.org/16734
Reviewed-by: Duncan Laurie <dlaurie@chromium.org>
Reviewed-by: Matt DeVillier <matt.devillier@gmail.com>
Tested-by: build bot (Jenkins)
18 files changed