soc/intel/skylake: Select VBOOT_EC_SLOW_UPDATE if EC_GOOGLE_CHROMEEC is selected

VBOOT_EC_SLOW_UPDATE should be selected if EC_GOOGLE_CHROMEEC is used as
building coreboot with Chrome OS support & without Chrome EC gives a
build error in coreboot.

Change-Id: I77eed0e1bdc1ba49381b72e21b0e18f573cadff0
Signed-off-by: Naresh G Solanki <naresh.solanki@intel.com>
Reviewed-on: https://review.coreboot.org/17020
Tested-by: build bot (Jenkins)
Reviewed-by: Aaron Durbin <adurbin@chromium.org>
Reviewed-by: Paul Menzel <paulepanter@users.sourceforge.net>

src/soc/intel/skylake/Kconfig

@@ -74,7 +74,7 @@ config CHROMEOS
 	select CHROMEOS_RAMOOPS_DYNAMIC
 	select EC_SOFTWARE_SYNC if EC_GOOGLE_CHROMEEC
 	select SEPARATE_VERSTAGE
-	select VBOOT_EC_SLOW_UPDATE
+	select VBOOT_EC_SLOW_UPDATE if EC_GOOGLE_CHROMEEC
 	select VBOOT_OPROM_MATTERS
 	select VBOOT_SAVE_RECOVERY_REASON_ON_REBOOT
 	select VBOOT_STARTS_IN_BOOTBLOCK