board: ti: am335x: Choose CPSW or PRUSS configuration based on jumper setting

board: ti: am335x: Choose CPSW or PRUSS configuration based on jumper setting

The am335x-ice-v2 board's Ethernet ports can be configured
in 'MII' or 'RMII' mode to be connected to 'PRUSS' or 'CPSW'
Ethernet subsystems.

This patch sets the environment variable 'ice_mii' to
'mii' or 'rmii' accordingly. Based on that we choose the
appropriate board devicetree i.e. 'am335x-ice-v2.dtb' or
'am335x-ice-v2-prueth.dtb'.

Since there are 2 Ethernet ports with 2 modes, there can be 4
configurations but for now we consider both ports in different modes
to be an invalid configuration and prevent boot in that case.

Signed-off-by: Roger Quadros <rogerq@ti.com>
[Amjad: use overlay instead of using new am335x-ice-v2-prueth.dtb]
Signed-off-by: Amjad Ouled-Ameur <aouledameur@baylibre.com>
Reviewed-by: Tom Rini <trini@konsulko.com>
[trini: Make prueth_is_mii be marked __maybe_unused]
Signed-off-by: Tom Rini <trini@konsulko.com>