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