hw/misc/sifive_u_otp: Add backend drive support

hw/misc/sifive_u_otp: Add backend drive support

Add '-drive' support to OTP device. Allow users to assign a raw file
as OTP image.

test commands for 16k otp.img filled with zero:

$ dd if=/dev/zero of=./otp.img bs=1k count=16
$ ./qemu-system-riscv64 -M sifive_u -m 256M -nographic -bios none \
-kernel ../opensbi/build/platform/sifive/fu540/firmware/fw_payload.elf \
-d guest_errors -drive if=none,format=raw,file=otp.img

Signed-off-by: Green Wan <green.wan@sifive.com>
Reviewed-by: Bin Meng <bin.meng@windriver.com>
Tested-by: Bin Meng <bin.meng@windriver.com>
Acked-by: Alistair Francis <alistair.francis@wdc.com>
Message-id: 20201020033732.12921-3-green.wan@sifive.com
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>