pwm: Improve Kconfig help text

The Kconfig help text should help the user understand what functionality
is provided by an option. This is especially true for new subsystems. An
improved help text is provided by this commit in the hopes of clarifying
the usefulness of the PWM framework.

Signed-off-by: Thierry Reding <thierry.reding@avionic-design.de>
Acked-by: Borislav Petkov <bp@alien8.de>