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)

src/mainboard/google/reef/mainboard.c

@@ -18,6 +18,7 @@
 #include <boardid.h>
 #include <console/console.h>
 #include <device/device.h>
+#include <ec/ec.h>
 #include <nhlt.h>
 #include <soc/gpio.h>
 #include <soc/nhlt.h>
@@ -25,8 +26,6 @@
 #include <variant/ec.h>
 #include <variant/gpio.h>
 
-void mainboard_ec_init(void);
-
 static void mainboard_init(void *chip_info)
 {
 	int boardid;