phy/at8031: enable at8031 to work on interrupt mode

The at8031 can work on polling mode and interrupt mode.
Add ack_interrupt and config intr funcs to enable
interrupt mode for it.

Signed-off-by: Zhao Qiang <B45475@freescale.com>
Signed-off-by: David S. Miller <davem@davemloft.net>