dt-bindings: pwm: convert pwm-bcm2835 bindings to YAML

Convert the DT binding document for pwm-bcm2835 from .txt to YAML.

Signed-off-by: Stefan Wahren <stefan.wahren@i2se.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Reviewed-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>