tegra: Rework Tamonten support

This commit uses the common Tegra board implementation instead of
duplicating a lot of the code. In addition, the Plutux and Medcom
specific board files can be removed as the MMC/SD setup is common
among all Tamonten-based boards.

Signed-off-by: Thierry Reding <thierry.reding@avionic-design.de>
Acked-by: Stephen Warren <swarren@wwwdotorg.org>
Signed-off-by: Tom Warren <twarren@nvidia.com>