drivers/net/phy: add fixed-phy / fixed-link support

This patch adds support for having a "fixed-link" to some other MAC
(like some embedded switch-device).

For this purpose we introduce a new phy-driver, called "Fixed PHY".

Fixed PHY works only with CONFIG_DM_ETH enabled, since the fixed-link is
described with a subnode below ethernet interface.

Most ethernet drivers (unfortunately not all are following same scheme
for searching/attaching phys) are calling "phy_connect(...)" for getting
a phy-device.
At this point we link in, we search here for a subnode called "fixed-
link", once found we start phy_device_create(...) with the special phy-
id PHY_FIXED_ID (0xa5a55a5a).

During init the "Fixed PHY" driver has registered with this id and now
gets probed, during probe we get all the details about fixed-link out of
dts, later on the phy reports this values.

Signed-off-by: Hannes Schmelzer <hannes.schmelzer@br-automation.com>

Signed-off-by: Hannes Schmelzer <oe5hpm@oevsv.at>
Acked-by: Joe Hershberger <joe.hershberger@ni.com>
Reviewed-by: Christian Gmeiner <christian.gmeiner@gmail.com>