dt-bindings: cpufreq: mediatek: Add MediaTek CCI property

MediaTek Cache Coherent Interconnect (CCI) uses software devfreq module
for scaling clock frequency and adjust voltage.
The phandle could be linked between CPU and MediaTek CCI for some
MediaTek SoCs, like MT8183 and MT8186.

The reason we need the link status between cpufreq and MediaTek cci is
cpufreq and mediatek cci could share the same regulator in some MediaTek
SoCs. Therefore, to prevent the issue of high frequency and low voltage,
we need to use this to make sure mediatek cci is ready.

Signed-off-by: Rex-BC Chen <[e-mail]>
Reviewed-by: AngeloGioacchino Del Regno <[e-mail]>
Acked-by: Rob Herring <[e-mail]>
Signed-off-by: Viresh Kumar <[e-mail]>

Documentation/devicetree/bindings/cpufreq/cpufreq-mediatek.txt

···
 	       Vsram to fit SoC specific needs. When absent, the voltage scaling
 	       flow is handled by hardware, hence no software "voltage tracking" is
 	       needed.
+- mediatek,cci:
+	Used to confirm the link status between cpufreq and mediatek cci. Because
+	cpufreq and mediatek cci could share the same regulator in some MediaTek SoCs.
+	To prevent the issue of high frequency and low voltage, we need to use this
+	property to make sure mediatek cci is ready.
+	For details of mediatek cci, please refer to
+	Documentation/devicetree/bindings/interconnect/mediatek,cci.yaml
 - #cooling-cells:
 	For details, please refer to
 	Documentation/devicetree/bindings/thermal/thermal-cooling-devices.yaml