Add generic parallel-in/serial-out shift register GPIO driver.

This includes SPI compatible devices like SN74165 serial-out shift
registers and the SN65HVS88x series of industrial serializers that can
be read over the SPI bus and used for GPI (General Purpose Input).
Signed-off-by: NAndrew F. Davis <afd@ti.com>
Tested-by: NSean Nyekjaer <sean.nyekjaer@prevas.dk>
Signed-off-by: NLinus Walleij <linus.walleij@linaro.org>