dt-binding: spi: add NPCM FIU controller

Added device tree binding documentation for Nuvoton BMC
NPCM Flash Interface Unit(FIU) SPI master controller
using SPI-MEM interface.

Signed-off-by: Tomer Maimon <tmaimon77@gmail.com>
Link: https://lore.kernel.org/r/20190828142513.228556-2-tmaimon77@gmail.com


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