spi: Kconfig: Add TEGRA20_SLINK entry

Added TEGRA20_SLINK entry on Kconfig with help description.

Signed-off-by: Jagan Teki <jteki@openedev.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Cc: Tom Warren <twarren@nvidia.com>
Cc: Stephen Warren <swarren@nvidia.com>