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.com


Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>