pwm: bcm2835: Support for polarity setting via DT

This adds support for the third (optional) pwm cell to specify the
polarity, which is needed by display backlights for example.

Signed-off-by: Stefan Wahren <stefan.wahren@i2se.com>
Reviewed-by: Eric Anholt <eric@anholt.net>
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>