ARM: tegra: dts: add sflash controller dt entry

Nvidia's Tegra20 have the SPI (SFLASH) controller to
interface with spi flash device which is used for system
boot. Add DT entry for this controller.

Signed-off-by: Laxman Dewangan <ldewangan@nvidia.com>
[swarren: move sflash node to keep file sorted]
Signed-off-by: Stephen Warren <swarren@nvidia.com>