dt-bindings: timer: convert rockchip,rk-timer.txt to YAML

Convert Rockchip Timer dt-bindings to YAML.

Signed-off-by: Ezequiel Garcia <ezequiel@collabora.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Link: https://lore.kernel.org/r/20210506111136.3941-4-ezequiel@collabora.com