tegra2: Add Avionic Design Plutux support.

The Plutux is a set-top box device based on the Tamonten processor
module. It can be connected to a display via an HDMI output.

Changes in v3:
  * Remove unused implementation of gpio_config_uart().
  * Implement MMC/SD card detection.
  * Drop board_mmc_getcd() which is now implemented by common Tegra2
    code.
  * Add MAINTAINERS entry.

Changes in v2:
  * No longer override the default CONFIG_SYS_TEXT_BASE setting.

Signed-off-by: Thierry Reding <thierry.reding@avionic-design.de>
Acked-by: Simon Glass <sjg@chromium.org>
Signed-off-by: Tom Warren <twarren@nvidia.com>