chromeec: Add event methods for EC requested throttle

Two new events possible from the EC for starting and stopping throttle.

These are handled in a per-board method that is defined under the
thermal zone.  This is not quite where I wanted it but the scoping
rules in ACPI don't let me have a defined external object in the
same scope.

Change-Id: I766f07b4365b29df3daa8e45e88f7c38c645c287
Signed-off-by: Duncan Laurie <dlaurie@chromium.org>
Reviewed-on: https://gerrit.chromium.org/gerrit/63988
Reviewed-by: Aaron Durbin <adurbin@chromium.org>
Reviewed-on: http://review.coreboot.org/4415
Tested-by: build bot (Jenkins)
Reviewed-by: Patrick Georgi <patrick@georgi-clan.de>

src/ec/google/chromeec/ec_commands.h

@@ -239,6 +239,11 @@ enum host_event_code {
 	/* Shutdown due to battery level too low */
 	EC_HOST_EVENT_BATTERY_SHUTDOWN = 17,
 
+        /* Suggest that the AP throttle itself */
+        EC_HOST_EVENT_THROTTLE_START = 18,
+        /* Suggest that the AP resume normal speed */
+        EC_HOST_EVENT_THROTTLE_STOP = 19,
+
 	/*
 	 * The high bit of the event mask is not used as a host event code.  If
 	 * it reads back as set, then the entire event mask should be