plat/sgm: Always use SCMI for SGM platforms

As on SGI platforms, SCPI is unsupported on SGM platforms.

Change-Id: I556ed095b9eb55b72447230ee2725d3c76160a08
Signed-off-by: Chris Kay <chris.kay@arm.com>

plat/arm/css/sgm/sgm-common.mk

@@ -4,6 +4,8 @@
 # SPDX-License-Identifier: BSD-3-Clause
 #
 
+CSS_USE_SCMI_SDS_DRIVER	:=	1
+
 CSS_SGM_BASE		:=	plat/arm/css/sgm
 
 PLAT_INCLUDES		:=	-I${CSS_SGM_BASE}/include