gpio: Add generic serializer DT binding

Add binding for generic parallel-in/serial-out shift register devices
used as GPIO.

Signed-off-by: Andrew F. Davis <afd@ti.com>
Acked-by: Rob Herring <robh@kernel.org>
[Clarified ngpios semantic]
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>