ARM: tegra: delete nvidia,tegra20-spi.txt binding

This binding shouldn't exist; Tegra20 has two forms of SPI controller
that are documented separately in nvidia,tegra20-sflash.txt and
nvidia,tegra20-slink.txt.

Signed-off-by: Stephen Warren <swarren@nvidia.com>
Reviewed-by: Thierry Reding <treding@nvidia.com>
Signed-off-by: Rob Herring <rob.herring@calxeda.com>