net: macb: add user io config data structure

Different implementation of USER IO register needs different
mapping for bit fields of this register. Add implementation
for this and, since clken is part of USER IO and it needs to
be activated based on per SoC capabilities, add caps in
macb_config where clken specific information needs to be filled.

Signed-off-by: Claudiu Beznea <claudiu.beznea@microchip.com>