pwm: bcm2835: Add support for suspend/resume

Similar to other drivers, we need to make sure that the clock is
disabled during suspend and re-enabled during resume.

Reported-by: Angus Clark <angus.clark@broadcom.com>
Signed-off-by: Florian Fainelli <florian.fainelli@broadcom.com>
Reviewed-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>