arm: mvebu: add LEDs support to defconfig file

The OpenBlocks AX3-4 platform has several LEDs, so it sounds wise to
enable LED support in mvebu_defconfig. We anticipate that more
platforms using Marvell EBU SoCs will have LEDs in the future.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Jason Cooper <jason@lakedaemon.net>