dt-bindings: timestamp: restrict node name suffixes

Make the pattern matching node names a bit stricter to improve DTS
consistency.  The pattern is restricted to -N suffixes to decimal
numbers.

Suggested-by: Rob Herring <robh@kernel.org>
Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Reviewed-by: Tony Lindgren <tony@atomide.com>
Acked-by: Rob Herring <robh@kernel.org>
Acked-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Reviewed-by: Conor Dooley <conor.dooley@microchip.com>
Link: https://lore.kernel.org/r/20230530144851.92059-7-krzysztof.kozlowski@linaro.org


Signed-off-by: Rob Herring <robh@kernel.org>