pwm: Add support for Broadcom iProc PWM controller

Add support for the PWM controller present in Broadcom's iProc family of
SoCs. It has been tested on the  Northstar+ bcm958625HR board.

Signed-off-by: Yendapally Reddy Dhananjaya Reddy <yendapally.reddy@broadcom.com>
[thierry.reding@gmail.com: bunch of coding style fixes, cleanups]
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>