hw/misc/sifive_u_otp: Add write function and write-once protection

hw/misc/sifive_u_otp: Add write function and write-once protection

 - Add write operation to update fuse data bit when PWE bit is on.
 - Add array, fuse_wo, to store the 'written' status for all bits
   of OTP to block the write operation.

Signed-off-by: Green Wan <green.wan@sifive.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Reviewed-by: Bin Meng <bin.meng@windriver.com>
Tested-by: Bin Meng <bin.meng@windriver.com>
Message-id: 20201020033732.12921-2-green.wan@sifive.com
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>