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>