arm64: dts: Add GPIO DT nodes for Stingray SOC

The GPIOs on Stingray SOC are based on iProc GPIOs hence
using this we add GPIO DT nodes for Stingray SOC.

Signed-off-by: Pramod Kumar <pramodku@broadcom.com>
Reviewed-by: Ray Jui <rjui@broadcom.com>
Reviewed-by: Scott Branden <sbranden@broadcom.com>
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>