Input: xpad - add support for leds on xbox 360 pad

Export LEDs on Xbox360 pad via led subsystem as a single device in
/sys/class/leds/xpad[0-9]+.

Xbox360 pad has four leds, which form a circle. Unfortunately the leds
can't be controlled independently and can only display a predefined
set of patterns (for example one is turned on wile others are off or
a rotating pattern - 1-2-3-4). To activate a pattern one needs to send
a specific command to the device (see http://www.free60.org/wiki/Gamepad).

Led subsystem allows us to set brightness, but there is nothing like
brightness on this device. So brightness is actually interpreted as
the command (only values between 0 and 14 are accepted).

Signed-off-by: Jan Kratochvil <honza@jikos.cz>
Signed-off-by: Dmitry Torokhov <dtor@mail.ru>