ARM: tegra: remove Whistler support

Whistler is an ancient Tegra 2 reference board. I may have been the only
person who ever used it with upstream software, and I've just recycled
the board hardware. Hence, it makes sense to remove support from software.

Signed-off-by: Stephen Warren <swarren@nvidia.com>
Acked-by: Mark Brown <broonie@kernel.org>
Signed-off-by: Thierry Reding <treding@nvidia.com>