iommu/rockchip: Play nice in multi-platform builds

The Rockchip IOMMU driver unconditionally executes code and registers a
struct iommu_ops with the platform bus irrespective of whether it runs
on a Rockchip SoC or not. This causes problems in multi-platform kernels
where drivers for other SoCs will no longer be able to register their
own struct iommu_ops or even try to use a struct iommu_ops for an IOMMU
that obviously isn't there.

The smallest fix I could think of is to check for the existence of any
Rockchip IOMMU devices in the device tree and skip initialization
otherwise.

This fixes a problem on Tegra20 where the DRM driver will try to use the
obviously non-existent Rockchip IOMMU.
Reported-by: Nicolas Chauvet <kwizart@gmail.com>
Cc: Heiko Stuebner <heiko@sntech.de>
Cc: Daniel Kurtz <djkurtz@chromium.org>
Reviewed-by: Heiko Stuebner <heiko@sntech.de>
Tested-by: Heiko Stuebner <heiko@sntech.de>
Signed-off-by: Thierry Reding <treding@nvidia.com>
Signed-off-by: Joerg Roedel <jroedel@suse.de>

drivers/iommu/rockchip-iommu.c

@@ -1015,8 +1015,15 @@ static struct platform_driver rk_iommu_driver = {
 
 static int __init rk_iommu_init(void)
 {
+	struct device_node *np;
 	int ret;
 
+	np = of_find_matching_node(NULL, rk_iommu_dt_ids);
+	if (!np)
+		return 0;
+
+	of_node_put(np);
+
 	ret = bus_set_iommu(&platform_bus_type, &rk_iommu_ops);
 	if (ret)
 		return ret;