dt-bindings: spi: tegra-sflash: Convert to json-schema

Convert the Tegra SFLASH controller bindings from the free-form text
format to json-schema.

Signed-off-by: Thierry Reding <treding@nvidia.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20230705152603.2514235-2-thierry.reding@gmail.com
Signed-off-by: Mark Brown <broonie@kernel.org>