net/macb: add support for resetting PHY using GPIO

With device tree it is no more possible to reset the PHY at board
level. Furthermore, doing in the driver allow to power down the PHY when
the network interface is no more used.

This reset can't be done at the PHY driver level. The PHY must be able to
answer the to the mii bus scan to let the kernel creating a PHY device.

The patch introduces a new optional property "phy-reset-gpios" inspired
from the one use for the FEC.

Signed-off-by: Gregory CLEMENT <gregory.clement@free-electrons.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>