pwm: visconti: Add Toshiba Visconti SoC PWM support

Add driver for the PWM controller on Toshiba Visconti ARM SoC.

Signed-off-by: Nobuhiro Iwamatsu <nobuhiro1.iwamatsu@toshiba.co.jp>
Reviewed-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
[thierry.reding@gmail.com: fix up a couple of checkpatch warnings]
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>