dt-bindings: timer: mediatek: Add CPUX System Timer and MT6795 compatible

Document the "CPUXGPT" CPU General Purpose Timer, used as ARM/ARM64
System Timer on MediaTek platforms and add the MT6795 compatible for it.
Signed-off-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Acked-by: Rob Herring <robh@kernel.org>
Reviewed-by: Matthias Brugger <matthias.bgg@gmail.com>
Link: https://lore.kernel.org/r/20220613133819.35318-2-angelogioacchino.delregno@collabora.comSigned-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>

Documentation/devicetree/bindings/timer/mediatek,mtk-timer.txt

 MediaTek Timers
 ---------------
 
-MediaTek SoCs have two different timers on different platforms,
+MediaTek SoCs have different timers on different platforms,
+- CPUX (ARM/ARM64 System Timer)
 - GPT (General Purpose Timer)
 - SYST (System Timer)
 
@@ -29,6 +30,9 @@ Required properties:
 	* "mediatek,mt7629-timer" for MT7629 compatible timers (SYST)
 	* "mediatek,mt6765-timer" for MT6765 and all above compatible timers (SYST)
 
+	For those SoCs that use CPUX
+	* "mediatek,mt6795-systimer" for MT6795 compatible timers (CPUX)
+
 - reg: Should contain location and length for timer register.
 - clocks: Should contain system clock.