leds: Introduce userspace LED class driver

This driver creates a userspace leds driver similar to uinput.

New LEDs are created by opening /dev/uleds and writing a uleds_user_dev
struct. A new LED class device is registered with the name given in the
struct. Reading will return a single byte that is the current brightness.
The poll() syscall is also supported. It will be triggered whenever the
brightness changes. Closing the file handle to /dev/uleds will remove
the leds class device.

Signed-off-by: David Lechner <david@lechnology.com>
Signed-off-by: Jacek Anaszewski <j.anaszewski@samsung.com>

Documentation/leds/uleds.txt

@@ -0,0 +1,36 @@
+Userspace LEDs
+==============
+
+The uleds driver supports userspace LEDs. This can be useful for testing
+triggers and can also be used to implement virtual LEDs.
+
+
+Usage
+=====
+
+When the driver is loaded, a character device is created at /dev/uleds. To
+create a new LED class device, open /dev/uleds and write a uleds_user_dev
+structure to it (found in kernel public header file linux/uleds.h).
+
+    #define LED_MAX_NAME_SIZE 64
+
+    struct uleds_user_dev {
+        char name[LED_MAX_NAME_SIZE];
+    };
+
+A new LED class device will be created with the name given. The name can be
+any valid sysfs device node name, but consider using the LED class naming
+convention of "devicename:color:function".
+
+The current brightness is found by reading a single byte from the character
+device. Values are unsigned: 0 to 255. Reading will block until the brightness
+changes. The device node can also be polled to notify when the brightness value
+changes.
+
+The LED class device will be removed when the open file handle to /dev/uleds
+is closed.
+
+Multiple LED class devices are created by opening additional file handles to
+/dev/uleds.
+
+See tools/leds/uledmon.c for an example userspace program.