net: stmmac: dwmac-sun8i: Add a shutdown callback

The Ethernet MAC and PHY are usually major consumers of power on boards
which may not be able to fully power off (those with no PMIC). Powering
down the MAC and internal PHY saves power while these boards are "off".

Reviewed-by: Chen-Yu Tsai <wens@csie.org>
Signed-off-by: Samuel Holland <samuel@sholland.org>
Signed-off-by: David S. Miller <davem@davemloft.net>