ARM: Kirkwood: increase atomic coherent pool size

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.

Suggested-by: Josh Coombs <josh.coombs@gmail.com>
Signed-off-by: Marek Szyprowski <m.szyprowski@samsung.com>
Acked-by: Jason Cooper <jason@lakedaemon.net>