dt-bindings: pwm: restrict node name suffixes

Make the pattern matching node names a bit stricter to improve DTS
consistency.  The pattern is restricted to:
1. Only one unit address or one -N suffix,
2. -N suffixes to decimal numbers.

Suggested-by: Rob Herring <robh@kernel.org>
Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Acked-by: Rob Herring <robh@kernel.org>
Reviewed-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Reviewed-by: Tony Lindgren <tony@atomide.com>
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>