tegra: remove support for UART SPI switch

This feature was only used for tegra20 seaboard that had a pinmux
conflict on the SPI pins.  These boards were never manufactured, so
remove this support to clean up SPI driver.

Signed-off-by: Allen Martin <amartin@nvidia.com>
Signed-off-by: Tom Warren <twarren@nvidia.com>
Reviewed-by: Stephen Warren <swarren@nvidia.com>