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>