spi: Add bindings for Intel Thunder Bay SOC

Add documentation for SPI controller in Intel Thunder Bay SoC.

Signed-off-by: Nandhini Srikandan <nandhini.srikandan@intel.com>
Acked-by: Rob Herring <robh@kernel.org>
Reviewed-by: Serge Semin <fancer.lancer@gmail.com>
Link: https://lore.kernel.org/r/20220713042223.1458-3-nandhini.srikandan@intel.com


Signed-off-by: Mark Brown <broonie@kernel.org>