spi: tegra: add spi driver for sflash controller

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

Signed-off-by: Laxman Dewangan <ldewangan@nvidia.com>
Acked-by: Stephen Warren <swarren@nvidia.com>
Tested-by: Stephen Warren <swarren@nvidia.com>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>