riscv: Return vm_paddr_t in pmap_early_vtophys()

We can't use a u_int to compute the physical address in
pmap_early_vtophys(). Our int is 32-bit, but the physical address is
64-bit. This works fine if everything lives in below 0x100000000, but as
soon as it doesn't this breaks.

MFC after: 1 week
Sponsored by: Axiado