arm: mvebu: Add Allied Telesis x240 board

The x240 and SE240 are a series of L2+ switches from Allied Telesis.
There are a number of them in the range but as far as U-Boot is
concerned all the CPU block components are the same so there's only one
board defined.

Signed-off-by: Chris Packham <judge.packham@gmail.com>
Reviewed-by: Stefan Roese <sr@denx.de>