net: emac: add support for device-tree based PHY discovery and setup

This patch adds glue-code that allows the EMAC driver to interface
with the existing dt-supported PHYs in drivers/net/phy.

Because currently, the emac driver maintains a small library of
supported phys for in a private phy.c file located in the drivers
directory.

The support is limited to mostly single ethernet transceiver like the:
CIS8201, BCM5248, ET1011C, Marvell 88E1111 and 88E1112, AR8035.

However, routers like the Netgear WNDR4700 and Cisco Meraki MX60(W)
have a 5-port switch (AR8327N) attached to the EMAC. The switch chip
is supported by the qca8k mdio driver, which uses the generic phy
library. Another reason is that PHYLIB also supports the BCM54610,
which was used for the Western Digital My Book Live.

This will now also make EMAC select PHYLIB.

Signed-off-by: Christian Lamparter <chunkeey@googlemail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>