arm64: dts: Add PWM DT node for NS2

Add device tree entry for PWM support for Broadcom Northstar 2 SoC.

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