gpio: Add driver for SPI serializers

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: Andrew F. Davis <afd@ti.com>
Tested-by: Sean Nyekjaer <sean.nyekjaer@prevas.dk>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
diff --git a/drivers/gpio/Kconfig b/drivers/gpio/Kconfig
index f52b259..d003abe 100644
--- a/drivers/gpio/Kconfig
+++ b/drivers/gpio/Kconfig
@@ -1048,6 +1048,12 @@
 	  SPI driver for Freescale MC33880 high-side/low-side switch.
 	  This provides GPIO interface supporting inputs and outputs.
 
+config GPIO_PISOSR
+	tristate "Generic parallel-in/serial-out shift register"
+	help
+	  GPIO driver for SPI compatible parallel-in/serial-out shift
+	  registers. These are input only devices.
+
 endmenu
 
 menu "SPI or I2C GPIO expanders"