dt-binding: spi: Add npcm845 compatible to npcm-fiu document

Add a compatible string and description for Nuvoton BMC NPCM845 FIU.

Signed-off-by: Tomer Maimon <tmaimon77@gmail.com>
Link: https://lore.kernel.org/r/20220718081146.256070-3-tmaimon77@gmail.com


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