dt-binding: Add ngpios property to GPIO controller node

Add ngpios property to the gpio controller's DT node so that controller
driver extracts total number of in-use gpio lines from DT and removes
dependency on driver.

Signed-off-by: Pramod Kumar <pramodku@broadcom.com>
Reviewed-by: Ray Jui <rjui@broadcom.com>
Reviewed-by: Scott Branden <sbranden@broadcom.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>