net: tsec: add option to set device max-speed via dts

Current tsec adapter sets adapter gigabit capabilities by default, and in
reality this must not always be the case.
It is possible that tsec adapter is used for 100Mbps connection, and in
this case setting 1000Mbps capabilities can lead to some side effects such
longer autoneg process.

In our ls102x designs this problem leads to long autoneg times (> 4 sec)
in case board rgmii link is 100Mbps capable only.
Limiting the rgmii link capabilities provides faster and smoother
link establishment.

Signed-off-by: Aleksandar Gerasimovski <aleksandar.gerasimovski@hitachi-powergrids.com>
Reviewed-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Priyanka Jain <priyanka.jain@nxp.com>