net/macb: Update device tree binding for resetting PHY using GPIO

Instead of being at the MAC level the reset gpio preperty is moved at the
PHY child node level. It is still managed by the MAC, but from the point
of view of the binding it make more sense to be part of the PHY node.

This commit also fixes a build errors if GPIOLIB is not selected.

Signed-off-by: Gregory CLEMENT <gregory.clement@free-electrons.com>
Signed-off-by: David S. Miller <davem@davemloft.net>