net: designware: Support high memory nodes

Some platforms (such as the Lichee Pi 4A) have their dwmac device
addressable only in high memory space. Storing the node's base address
on 32 bits is not possible in such case.

Use platform's physical address type to store the base address.

Signed-off-by: Nils Le Roux <gilbsgilbert@gmail.com>
Cc: Andre Przywara <andre.przywara@arm.com>
Reviewed-by: Andre Przywara <andre.przywara@arm.com>