hw/riscv: Always build riscv_hart.c

hw/riscv: Always build riscv_hart.c

Every RISC-V machine needs riscv_hart hence there is no need to
have a dedicated Kconfig option for it. Drop the Kconfig option
and always build riscv_hart.c.

Signed-off-by: Bin Meng <bin.meng@windriver.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Message-Id: <1599129623-68957-11-git-send-email-bmeng.cn@gmail.com>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>