ARM: tegra: fix colibri_t20 machine type

A while ago I got Russell to change the machine type of our Colibri T20
from COLIBRI_TEGRA2 to COLIBRI_T20 which at least in parts is also
reflected in his machine registry:

http://www.arm.linux.org.uk/developer/machines/list.php?id=3323



For us it is really very beneficial to actually still be able to boot
downstream L4T kernel with its working hardware accelerated
graphics/multimedia stack albeit it being proprietary/closed-source.

Signed-off-by: Marcel Ziswiler <marcel@ziswiler.com>
Acked-by: Stephen Warren <swarren@nvidia.com>
Signed-off-by: Tom Warren <twarren@nvidia.com>