mvebu: add support for WRT32X (venom)

Linksys WRT32X (Venom) is identical in hardware to the WRT3200ACM
with a different flash layout and boots zImage rather than uImage.

Specification:
- Marvell Armada 385 88F6820 (2x 1.8GHz)
- 256MB of Flash
- 512MB of RAM
- 2.4GHz (bgn) and 5GHz (an+ac wave 2)
- 4x 1Gbps LAN + 1x 1Gbps WAN
- 1x USB 3.0 and 1x USB 2.0/eSATA (combo port)

Flash instruction:
Apply factory image via web-gui.

Signed-off-by: Michael Gray <michael.gray@lantisproject.com>