dt-bindings: spmi: spmi-mtk-pmif: Add compatible for MT8188

Add compatible string for the SPMI block on MT8188 SoC, which is
compatible with the one used on MT8195.

Acked-by: Rob Herring (Arm) <[e-mail]>
Reviewed-by: AngeloGioacchino Del Regno <[e-mail]>
Signed-off-by: Fei Shao <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Stephen Boyd <[e-mail]>
Acked-by: Stephen Boyd <[e-mail]>
Reviewed-by: AngeloGioacchino Del Regno <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Greg Kroah-Hartman <[e-mail]>

Documentation/devicetree/bindings/spmi/mtk,spmi-mtk-pmif.yaml

···
       - items:
           - enum:
               - mediatek,mt8186-spmi
+              - mediatek,mt8188-spmi
           - const: mediatek,mt8195-spmi
 
   reg: