ec/google/chromeec: provide common SMI handler helpers

The mainboards which use the Chrome EC duplicate the
same logic in the mainboard smi handler. Provide common
helper functions for those boards to utilize.

BUG=chrome-os-partner:54977

Change-Id: I0d3ad617d211ecbea302114b17ad700b935e24d5
Signed-off-by: Aaron Durbin <adurbin@chromium.org>
Reviewed-on: https://review.coreboot.org/15685
Reviewed-by: Furquan Shaikh <furquan@google.com>
Tested-by: build bot (Jenkins)
3 files changed