net: phy: mscc: Add possibilty to disable combined LED mode

By default the LED modes offer to combine two indicators like speed/link and
activity in one LED. In order to use a LED only for the first of the two
modes, the combined feature needs to be disabled.

In order to do this we introduce a boolean devicetree property
'vsc8531,led-[N]-combine-disable' and wire it up to the matching
bits in the LED behavior register.

Signed-off-by: Frieder Schrempf <frieder.schrempf@kontron.de>