ChiliSOM has 2 Ethernet subsystems with different types of possibly used
PHY interfaces (i.e. MII, RMII, GMII, RGMII). Current code configured
pinmux for RMII on 1st Ethernet subsystem and enabled Ethernet MAC with
1 slave for all boards which use ChiliSOM.

This change moves pinmux configuration of 1st Ethernet subsystem to
ChiliBoard description, as this is board-specific.
Signed-off-by: Marcin Niestroj <m.niestroj@grinn-global.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>