ARM: dts: NSP: Add PWM Support to DT

Add PWM support to the device tree for the Broadcom Northstar Plus SoC.

Signed-off-by: Yendapally Reddy Dhananjaya Reddy <yendapally.reddy@broadcom.com>
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>