pwm: rockchip: Move the configuration of polarity

It is usually possible to configure the polarity, cycle and duty all at
once, so that the polarity and cycle and duty are applied atomically.
Move it from rockchip_pwm_set_enable() into rockchip_pwm_config(), as
well as prepare for the next atomic update commit.

Signed-off-by: David Wu <david.wu@rock-chips.com>
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>