net: rswitch: Add support for split MII and SerDes

net: rswitch: Add support for split MII and SerDes

This IP does support operating MII and SerDes via different ports.
Currently, the driver assumes that MII and SerDes are always bound
together on the same port, but this may not be the case. Implement
support for controlling MII and SerDes separately.

While the change is extensive, the gist of the change is to pass
pointer to the selected port registers to MII or SerDes functions,
depending on which port and operations should be done on that port.
Each combined ETHA instance contains both MII and SerDes register
pointers, which may not point to the same port, and passes those
registers to MII and SerDes functions respectively to control the
MII or SerDes of each port.

Signed-off-by: Marek Vasut <marek.vasut+renesas@mailbox.org>