stmmac: clean up platform/of_match data retrieval

Refactor code to clearly separate probing non-dt versus dt. In the
non-dt case platform data must be supplied to probe successfully.
For dt the platform data structure is created and match data is
copied into it. Note that support for supplying platform data in
dt from AUXDATA is dropped as no users in mainline does this.

This change will allow dt dwmac-* drivers to call the config_dt()
function from probe to create the needed platform data struct and
retrieve common dt properties.

Signed-off-by: Joachim Eastwood <manabian@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>