This adds the freq and duty_u16 keyword settings to the constructor, and
sometimes other details in the PWM section.
For mimxrt a clarification regarding the PWM invert argument was added, and
for rp2 a few words were spent on PWM output pairs of a channel/slice.
In that case, no Pin will be configured for the CS signal, even if it is
internally still generated. That setting allows to use any pin for CS,
which then must be controlled by the Python script.
Also make the default cs=-1 to match other ports (software CS).
