net: stmmac: enable Intel mGbE 2.5Gbps link speed

net: stmmac: enable Intel mGbE 2.5Gbps link speed

The Intel mGbE supports 2.5Gbps link speed by increasing the clock rate by
2.5 times of the original rate. In this mode, the serdes/PHY operates at a
serial baud rate of 3.125 Gbps and the PCS data path and GMII interface of
the MAC operate at 312.5 MHz instead of 125 MHz.

For Intel mGbE, the overclocking of 2.5 times clock rate to support 2.5G is
only able to be configured in the BIOS during boot time. Kernel driver has
no access to modify the clock rate for 1Gbps/2.5G mode. The way to
determined the current 1G/2.5G mode is by reading a dedicated adhoc
register through mdio bus. In short, after the system boot up, it is either
in 1G mode or 2.5G mode which not able to be changed on the fly.

Compared to 1G mode, the 2.5G mode selects the 2500BASEX as PHY interface and
disables the xpcs_an_inband. This is to cater for some PHYs that only
supports 2500BASEX PHY interface with no autonegotiation.

v2: remove MAC supported link speed masking
v3: Restructure  to introduce intel_speed_mode_2500() to read serdes registers
    for max speed supported and select the appropritate configuration.
    Use max_speed to determine the supported link speed mask.

Signed-off-by: Voon Weifeng <weifeng.voon@intel.com>
Signed-off-by: Michael Sit Wei Hong <michael.wei.hong.sit@intel.com>
Reviewed-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Signed-off-by: David S. Miller <davem@davemloft.net>