arm: mvebu: enable mini-PCIe connectors on Armada 370 RD

The Armada 370 RD board has two internal mini-PCIe connectors. This
commit adds the necessary Device Tree informations to enable the usage
of those mini-PCIe connectors.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Cc: Florian Fainelli <florian@openwrt.org>
Signed-off-by: Jason Cooper <jason@lakedaemon.net>