dt-bindings: pwm: Avoid selecting schema on node name match

Currently any node whose name starts with the "pwm-" prefix will match
this schema and in turn required the "#pwm-cells" property. Avoid this
by marking the schema with select: false, therefore only activating the
schema when directly included from a PWM controller schema file.

Signed-off-by: Thierry Reding <treding@nvidia.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>