spi: dw-apb-ssi: Add Intel Keem Bay support

Document Intel Keem Bay SPI controller DT bindings.

Signed-off-by: Wan Ahmad Zainie <wan.ahmad.zainie.wan.mohamad@intel.com>
Link: https://lore.kernel.org/r/20200505130618.554-7-wan.ahmad.zainie.wan.mohamad@intel.com


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