qti/sc7180: Fix GIC-600 support setting

The patch adding platform support for sc7180 landed around roughly the
same time as the patch that changed GICV3_IMPL to GICV3_SUPPORT_GIC600.
Thus the sc7180 Makefile is still using the old variable name which now
no longer does anything, and it hangs on boot due to the lacking GIC-600
support. This patch fixes the issue.
Signed-off-by: Julius Werner <jwerner@chromium.org>
Change-Id: Id76ada1445c3c5ac9a5a3697b4e749088b89d796

plat/qti/sc7180/platform.mk

@@ -79,7 +79,7 @@ include lib/coreboot/coreboot.mk
 PSCI_SOURCES		:=	plat/common/plat_psci_common.c				\
 
 # GIC-600 configuration
-GICV3_IMPL			:=	GIC600
+GICV3_SUPPORT_GIC600	:=	1
 # Include GICv3 driver files
 include drivers/arm/gic/v3/gicv3.mk