[ARM] Kirkwood: define SATA LED for netspace_v2

This patch add a GPIO LED named "ns_v2:blue:sata" which can be used to
enable or disable SATA activity LED blinking.

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