dt-bindings: timestamp: Deprecate nvidia,slices property

The property is not necessary as it is a constant value and can be
hardcoded in the driver code.

Signed-off-by: Dipen Patel <dipenp@nvidia.com>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Acked-by: Rob Herring <robh@kernel.org>