dt-bindings: timer: Add Mstar MSC313e timer devicetree bindings documentation

This adds the documentation for the devicetree bindings of the Mstar
MSC313e timer driver, found from MSC313e SoCs and newer.

Signed-off-by: Romain Perier <romain.perier@gmail.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20211217195727.8955-5-romain.perier@gmail.com


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