pinctrl: iproc: Add NSP and Stingray GPIO support

The iProc GPIO controller is shared among multiple iProc based SoCs.
In the NSP integration, the drive strength pinctrl function is
disabled. In the integration of Stingray, pinctrl is handled by another
block and this GPIO controller is solely used as a GPIO controller, and
therefore should not be registered to the pinconf framework

This patch introduces new SoC specific compatible strings
"brcm,iproc-nsp-gpio" for NSP with drive strength feature disabled and
"brcm,iproc-stingray-gpio" for Stingray with all PINCONF features
disabled

This patch is developed based on the initial work from Yendapally
Reddy Dhananjaya <yendapally.reddy@broadcom.com> who attempted to
disable drive strength configuration for the iProc based NSP chip. In
addition, Pramod Kumar <pramod.kumar@broadcom.com> also contributed to
make the support more generic across all currently supported PINCONF
functions in the iProc GPIO/PINCONF driver

Signed-off-by: Pramod Kumar <pramodku@broadcom.com>
Signed-off-by: Ray Jui <ray.jui@broadcom.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>