net: pcs: add 2500BASEX support for Intel mGbE controller

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