leds: Add driver for Acer Iconia Tab A500

Acer Iconia Tab A500 is an Android tablet device which has two LEDs
embedded into the Power Button. Orange LED indicates "battery charging"
status and white LED indicates "wake-up/charge-done" status. The new LED
driver provides control over both LEDs to userspace.

Signed-off-by: Dmitry Osipenko <digetx@gmail.com>
Signed-off-by: Pavel Machek <pavel@ucw.cz>