target/riscv: cpu: Set reset vector based on the configured property value

Now that we have the newly introduced 'resetvec' property in the
RISC-V CPU and HART, instead of hard-coding the reset vector addr
in the CPU's instance_init(), move that to riscv_cpu_realize()
based on the configured property value from the RISC-V machines.

Signed-off-by: Bin Meng <bin.meng@windriver.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Message-Id: <1598924352-89526-4-git-send-email-bmeng.cn@gmail.com>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>