leds: add LED driver for Network Space v2 LEDs

This patch add a LED class driver for the dual-GPIO LEDs found on the
Network Space v2 board (and parents). This include Internet Space v2,
Network Space (Max) v2 and d2 Network v2 boards.

This dual-GPIO LED is wired to a CPLD and can blink in relation with the
SATA activity. The driver expose this capability through a "sata" sysfs
attribute.

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