memory: tegra: Adapt to Tegra20 device-tree binding changes

The tegra20-mc device-tree binding has been changed, GART has been
squashed into Memory Controller and now the clock property is mandatory
for Tegra20, the DT compatible has been changed as well. Adapt driver to
the DT changes.

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