drivers: Add Ethernet PHY API

This commit adds support for Ethernet PHY drivers via a PHY API.

It also includes a driver for a generic MII compliant PHY
which supports most PHYs on the market.

Separating PHY driver from the SoC specific Ethernet driver
simplifies the Ethernet driver code and enables code re-use.
Drivers for specific PHYs with more advanced features, such as
RGMII delay in PHY can be developed independent of the Ethernet
MAC driver.

Signed-off-by: Arvin Farahmand <arvinf@ip-logix.com>