ARM: tegra: colibri_t20: iris: simplify model and compatible properties

Simplify model and compatible by dropping the 256/512 MB from the model,
-512 from the compatible and rename that property from toradex,iris to
toradex,colibri_t20-iris to be more in-line with all our other device
trees.

Signed-off-by: Marcel Ziswiler <marcel.ziswiler@toradex.com>
Signed-off-by: Thierry Reding <treding@nvidia.com>