gpio: Add a GPIO config

At present we have SPL_GPIO and TPL_GPIO but not piain GPIO. This
works because there is a special build rule in Makefile that always
includes the drivers/gpio directory.

It is better to have all driver directories included by drivers/Makefile
and there is already a rule in there for this purpose. It just needs a
Kconfig for U-Boot proper, so add one.

Enable the option always for now, since this mimics current behaviour.
This can be updated once DM_GPIO is used everywhere.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/drivers/gpio/Kconfig b/drivers/gpio/Kconfig
index ab9adbd..afb4c69 100644
--- a/drivers/gpio/Kconfig
+++ b/drivers/gpio/Kconfig
@@ -2,7 +2,19 @@
 # GPIO infrastructure and drivers
 #
 
-menu "GPIO Support"
+menuconfig GPIO
+	bool "GPIO support"
+	default y
+	help
+	  Enable support for GPIOs (General-purpose Input/Output) in U-Boot.
+	  GPIOs allow U-Boot to read the state of an input line (high or
+	  low) and set the state of an output line. This can be used to
+	  drive LEDs, control power to various system parts and read user
+	  input. GPIOs can be useful to enable a 'sign-of-life' LED,
+	  for example. Enable this option to build the drivers in
+	  drivers/gpio as part of an U-Boot build.
+
+if GPIO
 
 config DM_GPIO
 	bool "Enable Driver Model for GPIO drivers"
@@ -503,4 +515,4 @@
 	  into a number of banks each with 32 GPIOs. The GPIOs for a device are
 	  defined in the device tree with one node for each bank.
 
-endmenu
+endif