drivers: net: fsl_enetc: Add 2.5Gbps to supported link speeds

The original code enabled link speeds up to 1Gbps, but the interface can
go up to 2.5G, enable that speed to in PHY AN mask.

Signed-off-by: Alex Marginean <alexandru.marginean@nxp.com>
Acked-by: Joe Hershberger <joe.hershberger@ni.com>