Add YAML doc for the system tick counter which is present on Ralink SoCs.

cc: Daniel Lezcano <daniel.lezcano@linaro.org>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Sergio Paracuellos <sergio.paracuellos@gmail.com>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Link: https://lore.kernel.org/r/20231212093443.1898591-1-sergio.paracuellos@gmail.com