ARM: Orion5x: Fix too small coherent pool.

Some Orion5x devices allocate their coherent buffers from atomic
context. Increase size of atomic coherent pool to make sure such the
allocations won't fail during boot.

Signed-off-by: Andrew Lunn <andrew@lunn.ch>
Acked-by: Jason Cooper <jason@lakedaemon.net>
Signed-off-by: Olof Johansson <olof@lixom.net>