hw/riscv: Move sifive_u_prci model to hw/misc

hw/riscv: Move sifive_u_prci model to hw/misc

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_u_prci model to hw/misc directory.

Signed-off-by: Bin Meng <bin.meng@windriver.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Message-Id: <1599129623-68957-3-git-send-email-bmeng.cn@gmail.com>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>