dt-bindings: timer: Convert img,pistachio-gptimer to DT schema

Convert the ImgTec Pistachio Timer binding to DT schema format. It's a
straight-forward conversion.

Signed-off-by: Rob Herring (Arm) <robh@kernel.org>
Link: https://lore.kernel.org/r/20250506022249.2587839-1-robh@kernel.org


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