ARM: tegra: colibri_t20: simplify model and compatible properties

Simplify model and compatible by dropping the 256/512 MB from the model
and -512 from the compatible properties to be more in-line with all our
other device trees.

Signed-off-by: Marcel Ziswiler <marcel.ziswiler@toradex.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Thierry Reding <treding@nvidia.com>