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.