net: phy: fixed: support speeds of 2500 and 10000

Unlike the Linux fixed PHY driver, the one in U-Boot does not attempt to
emulate the clause 22 register set of a gigabit copper PHY driver
through the swphy framework. Therefore, the limitation of being unable
to support speeds higher than gigabit in fixed-link does not apply to
the U-Boot fixed PHY driver. This makes the fixed-link U-Boot
implementation more similar to the one from phylink, which can work with
any valid link speed.

Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Reviewed-by: Claudiu Manoil <claudiu.manoil@nxp.com>