hw/riscv/sifive_u: Connect the Cadence GEM Ethernet device

Connect the Cadence GEM ethernet device. This also requires us to
expose the plic interrupt lines.

Signed-off-by: Alistair Francis <alistair.francis@wdc.com>
Reviewed-by: Michael Clark <mjc@sifive.com>
diff --git a/default-configs/riscv32-softmmu.mak b/default-configs/riscv32-softmmu.mak
index 20e670d..7937c69 100644
--- a/default-configs/riscv32-softmmu.mak
+++ b/default-configs/riscv32-softmmu.mak
@@ -3,3 +3,5 @@
 CONFIG_SERIAL=y
 CONFIG_VIRTIO_MMIO=y
 include virtio.mak
+
+CONFIG_CADENCE=y