arm: mvebu: increase atomic coherent pool size for armada 370/XP

For Armada 370/XP we have the same problem that for the commit
cb01b633, so we applied the same solution: "The default 256 KiB
coherent pool may be too small for some of the Kirkwood devices, so
increase it to make sure that devices will be able to allocate their
buffers with GFP_ATOMIC flag"

Signed-off-by: Gregory CLEMENT <gregory.clement@free-electrons.com>
Acked-by: Marek Szyprowski <m.szyprowski@samsung.com>