spi: dt-bindings: mtk-snfi: Add compatible for MT7986

Add dt-bindings documentation of SPI NAND controller
for MediaTek MT7986 SoC platform. And add optional
nfi_hclk property which is needed for MT7986.

Signed-off-by: Xiangsheng Hou <xiangsheng.hou@mediatek.com>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Reviewed-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Link: https://lore.kernel.org/r/20230201020921.26712-4-xiangsheng.hou@mediatek.com


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