spi/tegra-sflash: assume CONFIG_OF, remove platform data

Tegra only supports, and always enables, device tree. Remove all ifdefs
and runtime checks for DT support from the driver. Platform data is
therefore no longer required. Rework the driver to parse the device tree
directly into struct tegra_sflash_data.

Signed-off-by: Stephen Warren <swarren@nvidia.com>
Signed-off-by: Grant Likely <grant.likely@secretlab.ca>