pinctrl: Update iProc GPIO DT bindings

Update the iProc GPIO binding document to add new compatible strings
"brcm,iproc-nsp-gpio" and "brcm,iproc-stingray-gpio" to support the
iProc based GPIO controller used in the NSP and Stingray SoCs,
respectively

Signed-off-by: Ray Jui <ray.jui@broadcom.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>