stmmac: add fixed-link device-tree support

In case DT is used, this change adds the ability to the stmmac driver to
detect a fixed-link PHY, instanciate it, and use it during
phy_connect().

Fixed link PHYs DT usage is described in:
Documentation/devicetree/bindings/net/fixed-link.txt

Signed-off-by: Mathieu Olivari <mathieu@codeaurora.org>
Signed-off-by: David S. Miller <davem@davemloft.net>