dt-bindings: timers: Add Ralink SoCs timer

Add YAML documentation for the timer which is present on Ralink SoCs.

Signed-off-by: Sergio Paracuellos <sergio.paracuellos@gmail.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Link: https://lore.kernel.org/r/20230620100231.1412582-1-sergio.paracuellos@gmail.com