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@xxxxxxxxxxxx>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@xxxxxxxxxx>