dt-binding: timer: document NPCM7xx timer DT bindings

Added device tree binding documentation for Nuvoton NPCM7xx timer.

Signed-off-by: Tomer Maimon <tmaimon77@gmail.com>
Acked-by: Rob Herring <robh@kernel.org>
Reviewed-by: Brendan Higgins <brendanhiggins@google.com>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>