spi: Add Socionext F_OSPI controller bindings

Add devicetree binding documentation for Socionext F_OSPI SPI flash
controller.

Signed-off-by: Kunihiko Hayashi <hayashi.kunihiko@socionext.com>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Link: https://lore.kernel.org/r/20221124003351.7792-2-hayashi.kunihiko@socionext.com
Signed-off-by: Mark Brown <broonie@kernel.org>