spi: dt-bindings: mediatek,spi-mt65xx: Add support for MT6991/MT8196 SPI

Add support for the SPI IPM controller found on MediaTek's MT6991
(Dimensity) and MT8196 (Kompanio) SoCs, with both having the same
controller IP, hence being fully compatible with each other.

Signed-off-by: AngeloGioacchino Del Regno <[e-mail]>
Link: https://patch.msgid.link/[e-mail]
Signed-off-by: Mark Brown <[e-mail]>

Documentation/devicetree/bindings/spi/mediatek,spi-mt65xx.yaml

···
           - const: mediatek,spi-ipm
       - items:
           - enum:
+              - mediatek,mt8196-spi
+          - const: mediatek,mt6991-spi
+      - items:
+          - enum:
               - mediatek,mt2701-spi
               - mediatek,mt2712-spi
               - mediatek,mt6589-spi
               - mediatek,mt6765-spi
               - mediatek,mt6893-spi
+              - mediatek,mt6991-spi
               - mediatek,mt7622-spi
               - mediatek,mt8135-spi
               - mediatek,mt8173-spi