Interfacing the MARVEL Chip on DE2-115

Yes you will need to write a MDIO master to be able to configure the PHY chip first. Note that the PHY datasheet isn't publicly available and you have to sign a rather restrictive NDA with Marvell if you want to get hold of it. You can go around it though and can have a look at the ALtera Nios dirver to see how the chip is initialized.

Then you can directly connect a custom design to the PHY's RGMII interface. Of course you will have to generate/detect the preamble part of the packet and handle all the Ethernet protocol yourself, and probably add a MAC/protocol filter on the input side because a PC is very talkative a lot when you connect something to its Ethernet port ;)