pinctrl: use ngpios propety from DT

Since identical hardware is used in several instances and every
instance will have different in-use pins. Hence extracting this
number from DT via "ngpios" property.

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: Linus Walleij <linus.walleij@linaro.org>