gpio/mockup: add virtual gpio device

This patch add basic structure of a virtual gpio device(gpio-mockup)
for testing gpio subsystem. The tester could manipulate such device
through userspace(sysfs or char device) and check the result from
debugfs.

Currently, it support one or more gpiochip(determined by module
parameters with base,ngpio pair). One could test the overlap of
different gpiochip and test the direction and/or output values of
these chips.

Signed-off-by: Kamlakant Patel <kamlakant.patel@broadcom.com>
Signed-off-by: Bamvor Jian Zhang <bamvor.zhangjian@linaro.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
diff --git a/drivers/gpio/Kconfig b/drivers/gpio/Kconfig
index 0e41c45..79429b9 100644
--- a/drivers/gpio/Kconfig
+++ b/drivers/gpio/Kconfig
@@ -279,6 +279,18 @@
 	  (EBU) found on Lantiq SoCs. The gpios are output only as they are
 	  created by attaching a 16bit latch to the bus.
 
+config GPIO_MOCKUP
+	tristate "GPIO Testing Driver"
+	depends on GPIOLIB
+	select GPIO_SYSFS
+	help
+	  This enables GPIO Testing driver, which provides a way to test GPIO
+	  subsystem through sysfs(or char device) and debugfs. GPIO_SYSFS
+	  must be selected for this test.
+	  User could use it through the script in
+	  tools/testing/selftests/gpio/gpio-mockup.sh. Reference the usage in
+	  it.
+
 config GPIO_MOXART
 	bool "MOXART GPIO support"
 	depends on ARCH_MOXART || COMPILE_TEST