board: add support for Minnowboard MAX

Very similar to the older Minnowboard, except that it works with mainline
Linux, uses 64bit firmware and a realtek NIC needing firmware.

The Linux configuration is based on the configuration fragment on elinux:
http://elinux.org/Minnowboard:MinnowMaxLinuxKernel



Many thanks to Circuitco for sponsoring a board.

Signed-off-by: Peter Korsgaard <peter@korsgaard.com>