arm: mvebu: add DTS file for Marvell RD-A370-A1 board

This patch adds the DTS file to support the Marvell RD-A370-A1
(Reference Design board) also known as RD-88F6710 board. It is almost
entirely similar to the DB-A370 board except that the first Ethernet PHY
is SGMII-wired and the second is a switch which is RGMII-wired.

Signed-off-by: Florian Fainelli <florian@openwrt.org>
Signed-off-by: Jason Cooper <jason@lakedaemon.net>