riscv: rv32: Root page table address can be larger than 32-bit

For RV32, the root page table's PPN has 22 bits hence its address
bits could be larger than the maximum bits that target_ulong is
able to represent. Use hwaddr instead.

Signed-off-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Signed-off-by: Palmer Dabbelt <palmer@sifive.com>