net: sun8i-emac: Determine pinmux based on SoC, not EMAC type

The pinmux choice for the RMII/RGMII pins the EMAC is connected to is
not dependent on the EMAC IP, but on the SoC it is integrated in.
Deriving the pinmux from the DT compatible string (as we do at the
moment) will thus cause problems with certain EMAC IP / SoC combinations.

To avoid this exact issue with the H616, let's use our Kconfig MACH
symbols to choose the correct pinmux setup.

Signed-off-by: Andre Przywara <andre.przywara@arm.com>
Tested-by: Jernej Skrabec <jernej.skrabec@siol.net>
Reviewed-by: Jernej Skrabec <jernej.skrabec@siol.net>