HID: add backlight support to PicoLCD device

Add backlight support to PicoLCD device.

Backlight support depends on backlight class and is only being
compiled if backlight class has been selected.

Signed-off-by: Bruno Prémont <bonbons@linux-vserver.org>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>