XPCS IP supports 2500BASEX as PHY interface. It is configured as
autonegotiation disable to cater for PHYs that does not supports 2500BASEX
autonegotiation.

v2: Add supported link speed masking.
v3: Restructure to introduce xpcs_config_2500basex() used to configure the
    xpcs for 2.5G speeds. Added 2500BASEX specific information for
    configuration.
v4: Fix indentation error
Signed-off-by: NVoon Weifeng <weifeng.voon@intel.com>
Signed-off-by: NMichael Sit Wei Hong <michael.wei.hong.sit@intel.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>