refactor(tc): append binding for SMMU-700

The usage for SMMU-700 is not consistent across TC platforms:

  SMMU-700 on TC2:

          | FVP   | FPGA
  --------+-------+------
  Display | Used  | Used
  GPU     | Used  | Used

  SMMU-700 on TC3:

          | FVP   | FPGA
  --------+-------+------
  Display | No    | No
  GPU     | Used  | No

This commit changes to use append mode for SMMU-700 to bind it on TC2
and TC3 separately. As a result, the TC_IOMMU_EN configuration is not
used, remove it.

Change-Id: Ic4152eb4c8ef97bf27b8a97c3c6cb86e32a2e8eb
Signed-off-by: Leo Yan <leo.yan@arm.com>
diff --git a/fdts/tc3.dts b/fdts/tc3.dts
index fe6a695..64546b6 100644
--- a/fdts/tc3.dts
+++ b/fdts/tc3.dts
@@ -85,4 +85,16 @@
 			shmem = <&cpu_scp_scmi_a2p &cpu_scp_scmi_p2a>;
 		};
 	};
+
+#if TARGET_FLAVOUR_FVP
+	smmu_700: iommu@3f000000 {
+		status = "okay";
+	};
+#endif
+
+	gpu: gpu@2d000000 {
+#if TARGET_FLAVOUR_FVP
+		iommus = <&smmu_700 0x200>;
+#endif
+	};
 };