Adding soil over the roots of a maple may injure the tree and eventually kill it.
At best, the roots of the maple will invade the bed and you still won't be able to grow anything. Maples are among the most challenging trees to grow anything under. Your best bet is a really tough groundcover--in my zone Liriope spicata (NOT L. muscari), Geranium macrorrhizum, vinca, or pachysandra would work, but they would take a long time to establish.

As laceyvail said, the maple roots will grow up into the raised bed. I had maple roots grow up into a container through the drain holes, and within just a few short years, I found that the entire container was filled with fine roots, right to the surface. I had to replace the soil entirely and put a saucer under the pot to keep the maple roots out. I also would be concerned with a raised bed against a fence causing rot, or if there is a small space between them, being difficult to weed the gap.

You could try any of laceyvail’s suggestions except Liriope, which isn’t hardy in MN. Other possibilities include Lamium/spotted deadnettle or Geranium Biokovo. Regardless, you will need to water well and frequently, not just the new plants, but also the entire area under the tree so that the tree roots aren’t growing toward pockets of moisture immediately around the new plants. Mulch the whole area lightly.

Clematis don’t like root competition, so near the maple won’t make it happy.