ARM: mvebu: add Linksys WRT1900AC (Mamba) support

The Linksys WRT1900AC (Mamba) is a router that has

- 2 mini-PCIe slots with Marvell 88W8864 radios
- 1 USB 3.0 port
- 1 USB 2.0/eSATAp port
- 2 Ethernet interfaces connected to a 88E6172 switch (1x WAN + 4x LAN)
- 128MB NAND flash
- 256MB RAM

gregory.clement@free-electrons.com: - add ARM to the title
	- fix the reference to CONFIG_DEBUG_MVEBU_UART0_ALTERNATE
	- fix the unbalanced comment for the syscfg partition

Signed-off-by: Imre Kaloz <kaloz@openwrt.org>
Acked-by: Gregory CLEMENT <gregory.clement@free-electrons.com>
Signed-off-by: Gregory CLEMENT <gregory.clement@free-electrons.com>