arm64: mvebu: Add Armada 3700 db-88f3720 development board support

This patch adds basic support for the Marvell Armada 3700 DB-88F3720
development board. Supported are the following interfaces:
- UART
- SPI (incl. SPI NOR)
- I2C
- Ethernet

Signed-off-by: Stefan Roese <sr@denx.de>
Cc: Nadav Haklai <nadavh@marvell.com>
Cc: Kostya Porotchkin <kostap@marvell.com>
Cc: Wilson Ding <dingwei@marvell.com>
Cc: Victor Gu <xigu@marvell.com>
Cc: Hua Jing <jinghua@marvell.com>
Cc: Terry Zhou <bjzhou@marvell.com>
Cc: Hanna Hawa <hannah@marvell.com>
Cc: Haim Boot <hayim@marvell.com>