dt-bindings: pinctrl: Add apple,npins property to apple,pinctrl

This property is used to describe the total number of pins on this
particular pinctrl hardware block.

Signed-off-by: Joey Gouly <joey.gouly@arm.com>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Acked-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20211026175815.52703-4-joey.gouly@arm.com
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>