hw/riscv: Move sifive_uart model to hw/char

hw/riscv: Move sifive_uart model to hw/char

This is an effort to clean up the hw/riscv directory. Ideally it
should only contain the RISC-V SoC / machine codes plus generic
codes. Let's move sifive_uart model to hw/char directory.

Signed-off-by: Bin Meng <bin.meng@windriver.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Message-Id: <1599129623-68957-9-git-send-email-bmeng.cn@gmail.com>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>