spi: Add Nuvoton WPCM450 Flash Interface Unit (FIU) bindings

spi: Add Nuvoton WPCM450 Flash Interface Unit (FIU) bindings

The Flash Interface Unit (FIU) is the SPI flash controller in the
Nuvoton WPCM450 BMC SoC. It supports four chip selects, and direct
(memory-mapped) access to 16 MiB per chip. Larger flash chips can be
accessed by software-defined SPI transfers.

The FIU in newer NPCM7xx SoCs is not compatible with the WPCM450 FIU.

Signed-off-by: Jonathan Neuschäfer <j.neuschaefer@gmx.net>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Link: https://lore.kernel.org/r/20221124191400.287918-2-j.neuschaefer@gmx.net
Signed-off-by: Mark Brown <broonie@kernel.org>