stmmac: add phy-handle support to the platform layer

On stmmac driver, PHY specification in device-tree was done using the
non-standard property "snps,phy-addr". Specifying a PHY on a different
MDIO bus that the one within the stmmac controller doesn't seem to be
possible when device-tree is used.

This change adds support for the phy-handle property, as specified in
Documentation/devicetree/bindings/net/ethernet.txt.

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