dt-bindings: pwm: mediatek,pwm-disp: Add compatible for MT6893

Add a compatible string for the Display Controller PWM IP found in
the MediaTek Dimensity 1200 (MT6893) SoC, which is compatible with
the one found in MT8183.

Signed-off-by: AngeloGioacchino Del Regno <[e-mail]>
Acked-by: Rob Herring (Arm) <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Uwe Kleine-König <[e-mail]>

Documentation/devicetree/bindings/pwm/mediatek,pwm-disp.yaml

···
           - const: mediatek,mt8173-disp-pwm
       - items:
           - enum:
+              - mediatek,mt6893-disp-pwm
               - mediatek,mt8186-disp-pwm
               - mediatek,mt8188-disp-pwm
               - mediatek,mt8192-disp-pwm