leds: add LED driver for Big Network series LEDs

This patch add a LED class driver for LEDs found on the LaCie 2Big and
5Big Network v2 boards. The LEDs are wired to a CPLD and are controlled
through a GPIO extension bus.

Signed-off-by: Simon Guinot <sguinot@lacie.com>
Signed-off-by: Nicolas Pitre <nico@fluxnic.net>