net: axienet: Re-initialize MDIO registers properly after reset

The MDIO clock divisor register setting was only applied on the initial
startup when the driver was loaded. However, this setting is cleared
when the device is reset, such as would occur when the interface was
taken down and brought up again, and so the MDIO bus would be
non-functional afterwards.

Split up the MDIO bus setup and enable into separate functions and
re-enable the bus after a device reset, to ensure that the MDIO
registers are set properly. This also allows us to remove direct access
to MDIO registers in xilinx_axienet_main.c and centralize them all in
xilinx_axienet_mdio.c.

Also, lock the MDIO bus lock around the device reset process, to avoid
MDIO accesses from occurring while the MDIO is disabled during the reset.

Signed-off-by: Robert Hancock <hancock@sedsystems.ca>
Signed-off-by: David S. Miller <davem@davemloft.net>