ARM: dts: tegra20: Update Memory Controller node to the new binding

Device tree binding of Memory Controller has been changed: GART has been
squashed into the MC, there are a new mandatory clock and #iommu-cells
properties, the compatible has been changed to 'tegra20-mc-gart'.

Signed-off-by: Dmitry Osipenko <digetx@gmail.com>
Acked-by: Thierry Reding <treding@nvidia.com>
Signed-off-by: Joerg Roedel <jroedel@suse.de>