tegra: Use SD write-protect GPIO on Tamonten

GPIO PI6 can be used to obtain the write-protect status of an SD card
inserted into the SD slot.

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>