dt-bindings: pwm: bcm2835: Increase pwm-cells

We need to increase the pwm-cells for the optional flags parameter,
before we can implement support for polarity setting via DT.

Signed-off-by: Stefan Wahren <stefan.wahren@i2se.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>