net: atlantic: A2: flow control support

This patch adds flow control support on A2.

Co-developed-by: Dmitry Bogdanov <dbogdanov@marvell.com>
Signed-off-by: Dmitry Bogdanov <dbogdanov@marvell.com>
Signed-off-by: Igor Russkikh <irusskikh@marvell.com>
Signed-off-by: David S. Miller <davem@davemloft.net>